While in theory your boss can't fire you because you like square-dancing, there are a lot of ways to fire someone arbitrarily without using the real reason. There's all sorts of dumb legal loopholes employers can use to fire someone they want gone. So if your square-dancing hating boss finds out you square dance then all they have to do is keep making your job worse and worse until you quit or find a superficial reason to fire you that will stand up in court - and there are a lot of those.
This is:
A) adorable
B) superawesomefanstico