It’s not because of what Kaepernick’s doing (kneeling during the US anthem, see above).
Or what any of the other protesters are doing either.
It’s because of the reactions the protestors are receiving. These range from firms ending sponsorship deals with players to schools threatening to discipline any students who dares copy Kaepernick’s example.
One soccer team even played the anthem early – before players took the field – to stop any potential protests. While there is, of course, at least one Congressman giving interviews calling Kaepernick “sympathetic to ISIS“.
You could argue those actions are no more inflammatory than the protests, but surely a real patriot is confident enough in their society to allow room for protest, especially when they’re continually trumpeting the “freedom” that society has?
The only good thing to say about the saga is that politicians haven’t passed any laws forcing people to stand yet, because that has happened before: in Japan, of all places. Japan is home to the world’s longest running anthem protest and if you want to learn about it, read my book, although I’ve just written an article for Foreign Policy magazine about the main protester – a lovely woman called Kimiko Nezu – that updates things and includes her views on Kaepernick. It also includes some quotes from a man who got beaten up for protesting India’s anthem.

Kimiko Nezy (on the right) celebrating in May after Japan’s Supreme Court ruled she shouldn’t have been suspended from her teaching job for six months for refusing to stand for the anthem. The ruling only covers a punishment in 2007, bizarrely, so she has many more cases to fight
Perhaps the only truly good thing to have happened in response to the Kaepernick saga so far is that South Park has satirised it. See below for a clip that includes a stadium announcer saying, “We now ask you all in solidarity to please rise, or sit, or take a knee, to honour America.” Very droll.