Generally speaking the US Constitution restricts the actions of government, and not anybody else. E.g. you can preach with a megaphone whatever you want on the sidewalk; you can't do it in my living room without my consent.

Also people tend to forget that freedom of speech is not freedom from consequences; limiting consequences of speech, even if it were desirable, can start restricting others' freedom of expression and association. If Bob gets on a stump and starts spewing controversial hate speech his employer is perfectly free to not want to associate with that behavior and fire him.