Actually, they're allowed to express political opinions. If you BS about it, nobody cares, and actually I do see a lot of opinions around. So just express is not a problem.
It only becomes a problem when the communists think you're "creating problem", or in their terms, "affect the harmony of society".
Personally, I don't neccessary like the western way of strong expressions that tends to really "affect the harmony of society" (as bad in Hong Kong sometimes). But unfortunately, they don't have a efficient way of letting opinions flow into the "brain" of the central government, and the central government is still having big problem controlling local governments (which is actually the biggest problem that avoid the central government to do anything "better").
Basically, most people always "express" their opinion not about central government, but local government (because federal government actions usually doesn't affect their life in a bad way, instead, often in a pretty good way these days, my opinion, at least). But those local provincial governments have their own court system and police who sometimes just go capture those people who try to go to Beijing to complain about local governments. Sometimes, they even cross the province boundary to stop those people.
Certainly a hard problem to solve. But it's really what most Chinese care most about. Federal freedom... I think it's much less of a concern, if you get right down to what exactly are the problems that people are having. At least, for now.