Teachers are expected to be very conservative. Parents don't want to send their kids to school if their teacher looks like someone who would have done drugs. Iguaranteeyou that that super strict teacher you had in high school probably got drunk off their ass on the weekends.
There are some exceptions, but in reality, teachers are supposed to be completely impartial, and not impose their views on students. If, by your appearance, you are generating an opinion, or what others might think is your opinion, you'll have a really really hard time finding a teaching job.
Now, is this right? I don't think so. I would have loved it if my teachers had their own opinions. It would have made school more educational, and more interesting. Kids don't have to listen or agree with them, they just have to learn. Seeing someone with a strong stance shows them that they too should have their own opinions. What the system does, now, is just teach kids that they should not think for themselves.