What if you dont like it or need to get your money back?
On the one hand, caveat emptor.
On the other hand, there's a reason forgiving return policies exist: they drive a shitload of business on "I probably can buuuuuuut" purchases, and Best Buy with its expensive consumer electronics is nothing but "I probably can buuuuuuut" purchases. Attempting to have their cake (advertising 30-day no-questions-asked returns) and eat it too (ban people when they take advantage) may help a little with the guys who "rent" HDTVs for the Super Bowl or laptops for business trips, but long-term trying to squeeze it is money out of their pocket.
Sure, Best Buy eats the open box discount if they let people return whenever, but now they've got a regular customer who isn't gonna buy ANYTHING because he's up shit creek if it's defective.