An NFL quarterback’s recent sideline protest is a reminder that for all of the beauty of the principles enshrined in the U.S. Constitution, the exercise of them in real life can often be offensive.

On Friday, Colin Kaepernick of the San Francisco 49ers refused to stand for the national anthem before a game, saying he would not “show pride in a flag for a country that oppresses black people and people of color.” His action, which garnered headlines and understandable ire, is another in the long tradition of athletes leveraging their sports spotlight to make a statement. The “Black Power” salute during the 1968 Olympics medal ceremony may well be the most memorable of these moments. And Muhammad Ali refused to fight in Vietnam, a decision he defended by decrying racism.

Kaepernick’s protest is a textbook example of a citizen exercising the freedom of expression that the First Amendment guarantees every American. He also did not violate NFL policy, which encourages rather than requires players to stand for “The Star-Spangled Banner.” And yet, his protest felt like a kick in the gut to many.

Here is someone who has become unbelievably wealthy in a system he is now decrying. His right to criticize that system — a right that should not be questioned — has nevertheless been paid for in blood many times over by those who fought for our nation. Although the military defends our values and our people — not a song or a flag — it was still stomach-churning for many Americans to watch Kaepernick disrespect these two treasured symbols.

One of the greatest achievements of this country is that freedoms aren’t reserved only for the wealthy or powerful. That means fans are free to have an opinion and, in turn, that means Kaepernick will have to face the consequences of his action. That’s likely to include lost endorsements, less lucrative contacts and, yes, some serious booing when the 49ers go on the road. The quarterback is lucky that his team’s schedule doesn’t include the Philadelphia Eagles, whose fans are notorious for their brutal razzing of opposing athletes.

But a cautionary note for Kaepernick’s critics is in order as the controversy continues. It’s fine to express opinions on social media or in news story comments. But it’s important to do so in a civil manner. The digital age’s new public forums regrettably have brought out the worst in some people. Facebook reaction to the Star Tribune’s Kaepernick story included profanity, a call for him to be deported and a number of people saying they hoped he’d be injured.

Whether it’s the Kaepernick protest or the political news of the day, no discussion is served by spitefulness or personal attacks. The Star Tribune and other media companies employ moderators to keep commenters civil on their websites. But self-policing on social media, especially with this fall’s election, is also in order. If you wouldn’t want your mom to see it, don’t post it.