It's petty, and ridiculous, IMO. I don't cheer against the Cubs unless the Sox are playing the Cubs. I don't ever cheer for a rival of the Cubs, just because they're the Cubs. None of that petty nonsense makes sense to me. It's still a Chicago team, and cheering for another city over the Cubs, just because they're the Cubs, is not gonna happen, at least for this Sox fan.
I wouldn't say "a lot of Sox fans enjoy the Cardinals winning" I would just say it's the petty, immature portion of the fanbase, and that portion is likely not even close to a majority.