Because it's not really a choice. You can pretend play to advocate for whatever political ideology. The same way you can have sex with women even if you're gay or vice versa. You can choose your sexual behavior.
But political orientation is not really a choice. It is perhaps a result of your choices, early influences, social group, etc.
It helps that I live in a country with more than 2 dominant political parties; it lets me think in shades of gray (rather than in black-and-white).
The idea is that you can believe whatever you want, but as soon as you start to propagate violence and pose an active threat against democracy - like e.g. making detailed plans to overthrow the government, advertising that only certain people should be allowed to vote, etc - there should be reasonable limits.
It's kind of a water a person swims in. It's the way he thinks. It's too meta for most people to even think about it as something chooseable.