I hate to burst your bubble but your thinking is very flawed. First a commission on a TV vs a Projo/Screen combo is vastly different. I won't get into how different but just know that TVs have the lowest commission of almost any product in all of CE save only for computers. To say nothing of what labor for the job would cost.
Second: if your basement is finished they may have thought running the video would have presented a problem or possibly there was some other kind of issue. If you went to a design center and has a walk with the designer and their project manager and they suggested a different solution I'm sure they're was a reason.
Third: fun fact projectors and screens have the highest return/exchange rate of ant class product in the industry. Often if a room isn't ideal or the end results are questionable; you'll be offered a less risky solution. Even if that translates to less money for the salesperson in the end.
Sorry for getting on a soapbox here but I feel I have to defend salesmen the world over. I've done sales for a long time (I'm a custom integrator now) and I have to say. Sales is a noble profession. You have to win your clients' trust and respect that trust by working with their best interests in mind. You have to support their dreams while being critical of the ideas that don't work.
Ok, there's a lot to deal with here. I'll start with some useful information for everyone who ever spends money on anything:
Profit margins on big-ticket electronics are thin. They are also fixed by a thing called MAPP pricing. (Manufacturer's Agreed Price Point). This is true in virtually every industry where the end-user is separated by one or more degrees from the manufacturer by distributors, wholesalers, retailers, etc. This is also why the same make and model of [device] is the same price regardless of where you buy it... and if it isn't, most retailers will "price match" the competition. (Basically because you, the customer, caught them trying to run the price up). It's also why EVERYONE advertises their prices as being "x% below MSRP!". If you pay MSRP for something, you're being robbed. If everyone follows MAPP pricing, profit margins are consistent and predictable.
Sales figures for individual employees are generally not related to the thickness of the profit margin, but listed in aggregate. So a $7000 TV looks better to the accounting department than a $30 cable... HOWEVER...
While it varies from company to company, almost all big-box retailers that keep track of sales peoples' totals are also actively pushing them to sell high-profit items. Extended warranties, installations, accessories... these are where the money is made. But after years of research, all these companies know (and can support with staggering quantities of research) that the guy who buys the $7000 TV is much more likely to drop an extra $250 on cables and accessories from the same store, and that $250 is 75%-90% profit. Given this, the strategy is simple: Stock the Big Bad TV and all its extra bits, and then sell it aggressively. Customers will do a lot of the work for you if you can get them on board with the Big Bad TV.
Projectors tend to violate the big-box retailers proven approach by requiring different accessories than their TV cousins... they need completely different mounting hardware, usually a different complement of cabling, and often a screen. Since these items are for a niche market, Best Buy et al don't often stock them... so if they DO sell a projector, the high-profit accessories are going to get purchased elsewhere. And as was pointed out, the proper setup of a projector is much more demanding which often results in a less-than-satisfied customer. Hence the high return rate. Given the risk of return and the difficulty in accessorizing for establishments like Best Buy, it makes perfect sense that a salesperson interested in keeping their job would push a customer towards a solution for which they are better equipped to provide all the necessary peripherals.
I'm not suggesting that following this line of reasoning makes a salesperson a bad human. What I AM suggesting is that the salesperson is much more likely to respond to pressure from their management than they are to loosely stated demands from a customer.
The OP stated in his post that he wanted a projector, and that the salesperson from Magnolia tried to talk him out of it in favor of a large and expensive TV that he didn't want. If the salesperson truly had the OP's needs in mind, he would have suggested products and solutions that fit the stated demands. I back this up with the OP's statement that the Magnolia Guy said "people could get up and their heads might be in the way of the projector. That's not a sales guy looking out for a customer in this case... that's a pretty weak argument against a product he doesn't want to sell.
So, my thinking is absolutely not flawed. Based on the available information, the OP was talking to a bad salesperson. He knew he didn't have a 35' HDMI cable in stock, nor a 20' IEC power cable. He may or may not have had a projector that would have worked in the OP's setting, but he didn't want to risk having it returned after OP accessorized elsewhere. So he didn't offer the best solution, he offered the one that made his job easy. Not cool.
Sales, done right, is a noble profession. A good salesperson is hard to find, and once found should be kept... but those are few and far between. You know you have a good one when they tell you to your face that the product you should have is one they don't sell, and then they follow that with where to get it. Unfortunately, while this may result in a happy customer, an unhappy manager is almost always the side effect. On top of that, being a good salesperson requires intimate knowledge of your product line... and I've never known Best Buy to keep a steady supply of well-informed sales people.