Tackiness of this being in the invitation aside, OP I think you're reading too much into this. They don't want or need gifts, yet they know they're going to get them. So they'd rather have the gifts go to charity. Like someone said it's no different from asking for charitable donations in leiu of funeral flowers. OK, so you don't like this particular charity. It's not likely to find a charity that EVERYONE on their guest list will like. Rather than be offended at your friends' opposing viewpoints, simply donate to another charity that you like. And preferably one that won't affect the couple. Or just give a card.
As to the tacky question, well I'm somewhat inclined to think this is ok. I don't like registry information in the the invitation, but this is a little different as the gifts are not actually going to the couple. And this is probably the best way for them to get the info out. I don't know, I'm torn on that issue.