Best Buy $50 gift cards can be spent on anything in store. Not much of a need to try to return them if you are going to spend money there at some point. Lots of people have talked about getting accessories there with their gift card too if Sprint did something like this.
I'm guessing Sprint just secured a bit of business back from 3rd party retailers. Even I'm thinking about trying to use the 21st (the catch is that my upgrade may or may not be ready for the 24th--or 21st. So I may have to try to call or go in store and force my hand or wait it out till the 1st of the month.)
Best Buy will probably do something to fight for their business back. I wonder what it will be this year? Last year they gave away free Plantronics Explorer 320 bluetooth earpiece's ($20 value) and offered double rewards points.