[joshuamorton]

Companies are just people as far as the constitution is concerned (or really they're groups of people, but assembly is also protected by the first amendment!)

The food safety labels is an interesting point, but I'm not even talking about gov regulations here. Just, let's say Twitter deletes your post. What do you sue them for?

The 1a allows twitters employees to express themselves as they wish, even through the company, so their removal of your post is simply their own protected expression.