Maybe because you're not American (right?) but you're conflating freedom of speech with constitutional rights.
If the guy who originally made the comment said "a blow to our First Amendment rights" then yeah, there's no First Amendment rights on Facebook.
But this country was founded by dissidents, and the notion of freedom of speech writ large goes far beyond what the First Amendment guarantees. It's a generalized ethos that we have a free and open society where freedom to say what you will is to be encouraged and never discouraged -- whether guaranteed by law or not. It's part of being a Good American.
At least, it was, until the PC cancer started spreading in the last 2 decades or so.