I’m all about reusable bags—I totally collect them and am a junkie and would buy them anyway.
With that said, it’s pretty crappy of them to quit offering plastic bags and make you buy a bag to carry your souvenirs in. People don’t go to a theme park (well, most don’t—I have foldable ones I bring in my pocket that are Disney!) with bags in hand to carry their souvenirs home in so they kind of have you in a corner. If they keep the price to about a buck, that’s pretty decent but going anything over that is pretty rotten of them!