Because claiming to be "nonpolitical" continues to mean wishing to be willfully ignorant and comfortably within the realm of political norms you're so used to that you just don't notice them.
People have a lot to say about my position on something. Should i examine some of my underlying assumptions and unexamined opinions regarding this issue?
I've had this exact conversation with an irl friend about how life is political, and you don't get to pretend that your worldview is somehow so above it all and apolitical so that you can tell other people to stop posting memes in the meme channel.