It's because they don't actually know what "Roe vs Wade" means. They've heard the news talk about it and have a vague idea, but all they know is that it's somehow related to abortion.

Even so, the analogy still doesn't make sense. The first domino represents something they like, and the rest of the dominoes represent something they don't like. But that's what happens when people feel the need to crap out political commentary as fast as possible without actually thinking.