Zero interest doesn't equal zero knowledge, I find a lot of things people like to be really boring but I usually have input on the topic anyway, therefore I comment.
To get a KICK out of it!
Just for fun.
Obviously they don't have 'zero interest' if they took the time to comment. I'm guessing you got/have seen comments where someone disagreed quite heavily with the OP. It happens man, get over it.
Maybe you want to get your opinion across.
Perhaps they have nothing better to do. I'm not saying it's a good reason, but...
@whereismygreatperhaps, but why would you want to get your opinion across if it's just irrelevant or you don't even care?
Maybe you want to get your opinion across