I've been on both sides of this discussion, and they're both difficult. As a B&M business owner you have to worry about margins, revenue per ft2, and gross profit per hour. Even if your sales staff is not commission, you still have to make the math work, on top of inventory, you have to cover rent, utilities, taxes, wages & benefits.
As a customer you want service and time without obligation. A good sales person needs to generate about $250 per hour in gross profit to make about $50k a year. Given overall margins (Video is as low as 10 points, Speakers up to 50 points; cables typically even higher) you can do the math and see that stores can only afford so much time for demos.
Time isn't just money, time is rent, groceries and the light bill. Respect their time and you'll get good service. Waste their time and , well, don't expect much in return for your .... Nothing.