The problem facing these NFL players is not that they are peacefully protesting, it's how and when they are doing it. They
are protesting the national anthem
and the flag - even though some are trying to say otherwise. Taking a knee during the anthem wasn't a coincidence, it was a statement from Kaepernick (and now many others) expressing that he didn't respect the anthem or flag (in essence - the country). Fair enough, I think he looks foolish and hypocritical but he has a right.
Anyway, many americans, fans and veterans love America, love the flag and anthem and have a deeper understanding of the symbolism that those things represent and a protest like this puts those people squarely in the middle of choosing to side with the players and their protest or the national anthem (representing the country). This decision becomes more complicated when the NFL owners, clearly terrified of being labelled racist and clearly hoping to not
**** off 65% of their team and the black community, cave in and hope for the best.
This means that the NFL fan is effectively supposed to choose between the protestors/NFL teams and their flag (representing their country). For many people, and especially veterans this is a nearly impossible decision, but they are painfully siding with the flag and staying away from the NFL. The problem with the manner of protest, is that it's driven away many people (millions) who otherwise might side with it. IOW, the protest itself is more of a factor than the issue.
In closing, it doesn't help that anyone who sides with the anthem/flag is now labelled a racist. The Pittsburgh penguins are already being labelled as such for accepting a visit to the Whitehouse.