It's Legal To Curse Out A Cop, But Cops Still Arrest People For It
There's that pesky First Amendment thingie, and as long as you aren't threatening the cop, your pottymouthisms are legal.

However, Terrell Jermaine Starr writes at Alternet:

Courts around the country consistently rule in favor of defendants who find themselves behind bars for using profane, but protected speech against law enforcement.

...While it is not known exactly how many of these arrests are the result of profanity being used toward police officers, it does reveal a trend of hyper-policing that troubles Sanders.

"As long as they are not breaking the law, people are allowed to have their First Amendment right to say, 'Fuck the police.' It's reality. 'Fuck the police. I hate all cops.' Is it a nice thing to hear? No. But is it legal? Sure it is. There is no law against that," he said.

via Jay J. Hector