Do you not see the hypocrisy?
As elderban pointed out, the first amendment is between the government and It's citizens, so there is some room for interpretation.
Either an employer can penalize use of speech, or the first amendment still protects it.
If employers can penalize speech, they can fire Roseanne for her tweets, but the NFL can also fine players for protesting.
If you believe the first amendment protects workers from retaliation from employers, then the players right to protest is protected, but so is Roseanne's tweet.
You could even argue that employers can limit speech, but only while someone is working and performing job related duties. Since the players are on the field and reasonably considered to be working and performing duties related to work, while Roseanne's tweet was not while working and performing duties related to work, you could argue that the players speech is not protected in that moment, but that Roseanne's tweet was.
At any rate, there is no logical interpretation that says the players protests is protected by the first amendment, but Roseanne's tweet wasn't. The first amendment doesn't differentiate between "acceptable" and "unacceptable speech". That is the whole point of the first amendment.