“ I don't understand why many liberals defend a corporations freedom to silence speech but get angry when conservatives in the government do it. ”

Because it’s a false equivalence that has nothing to do with being “liberal”.

The government has the power of coercion. No corporation has that by law.

In general, you are conflating the right to free speech with the privilege of easy armchair dissemination of one’s speech. Simply, your right to say anything doesn’t extend to forcing the world to disseminate and listen to what you have to say. Frankly, that extension is very much 1984.

In extremis, Trump has the means to publish a newsletter and nobody is stopping him from doing it.