Reflex. People's opinion on a subject changes if you tell them which political group supports it, sometimes even if they get asked twice in a row. Tribal identity determines ideology more than the other way around for a lot of people.

So as soon as Labour comes out for something, Cons are inclined to be against it and so on. The only way to have neutral protests is if no one visibly backs them and they don't become associated with a side, but then how do they get support and organization?