Is it illegal to misquote someone/misinterpret a meaning?

I heard that Dr. Seuss attempted to sue pro-life people for using a quote from "Horton Hears a Who" because they used it to promote pro-life, but that's not what the quote is about. But isn't that completely legal? They gave him credit for the quote, and they weren't trying to make money off of it. I was just wondering if it's legal to use something in that case. The way I've seen it, art, writing, and all of that is open to interpretation, and its okay if that interpretation is wrong.

I'm not trying to get into a pro-life vs. pro-choice or talk about Dr. Seuss in a good or bad way, I was just confused and wondering if it was legal or not.