To be honest this happens in a lot of places.
Gamestop will take in games disc only, and there is nothing on the barcode that says they can't accept the game for trade in.
And not every Gamestop employee will know about Redbox. Some of them can barely even dress themselves properly, so you can't expect every employee to be a genius, and knows what's what. And even if they knew they were taking it in, the seller is taking full responsibility that they will be taking in the fines for selling the game as opposed to returning it to Redbox. Gamestop employees are paid to know Gamestop's poliies (sometimes they don't even know that), not Redbox's.
The days of "Not For Resale" are basically over. Only times stores won't take in trades are usually if someone trades in a bulk of disc only games, or if a bunch of games are still sealed. Those are really the only exceptions. Pawn laws are also varied by state, and in the cases aforementioned wouldn't even apply.
And since you mentioned that they charges for people who "lose" their disc or sell it to Gamestop, is this not Redbox's policy to combat people who steal or sell the games?
Look, I'm not saying this is right in any possibility imaginable, but it doesn't really seem like it's something you should be pinning on one specific company alone just because you've only seen it happen there.
But to be honest this has been a problem for awhile, and at many other stores I've been too that's not just Gamestop. HPB is one, seen them at thrift stores too. It's a pretty common problem, and until Redbox decides that this structure they have put into place isn't working properly it is up to them to do something about it, not Gamestop.