"I was unaware that expressing an opinion was considered "anti-american", or "anti-western". "

It obviously depends on the opinion, and those doing the considering.

"If you were not allowed to express an opinion, wouldn't that be something very anti-american?"

Any political opinion is allowed (and/or not refereed) by the US govt. [Forms of political expression is allowed by the govt. Access to information is also part of this- though the State and citizens have certain rights in terms of privacy.]
I have the freedom to say or do anything- though there could be legal consequences [a lawyer could go on for years regarding possible legal consequences]. Freedom of speech is mostly related to political discussion. Which boils down to the freedom to change the laws. It's the recognition that humans (rather than some unknowable thing like the distant past, some really smart guy, or God) can create all the laws- at least in terms of governmental laws. Freedom is in terms of citizen vs the State, and pretty is silly unless you talking about some relationship to the State.
Anyways, Americans can certainly say that something is or is not anti-american. Having "rights and allowing" is about freedom from govt interference- an exclusion of State Power from citizens' relationship with each other- no one is being arrested or restricted by the govt for saying anything. We the people may or may NOT wish to listen some fruitcake spew his/her opinion. We may also express our displeasure at someone betraying public confidence [as can we cheer them when they pleases us]. Movie stars or other public fiqures have "public confidence". They have been "voted into office" and they can be "voted out of office". Impeachment being conducted by movie producers and more fundementally the "box office".