Well good ol' first amendment allows anyone to spew out their opinion about anyone's mom, whether you like it or not.
Ah, I guess you're not up to speed on common idioms, so allow me to help.
When English speakers use the phrase "you can't express an opinion" we are not meaning that you must literally become mute as you seem to have interpreted it. Rather, it's an expression meaning that an opinion is based on faulty information, and is thus not applicable or useful to the conversation.
You're likely to run into that phrase again in the future, so now you'll now what people mean when they say that.