Arrest

By definition, arrest is "to take or keep in custody by authority of law". They list other definitions but they matter less.

Although it's often necessary to arrest people who cause danger to persons or property, too often the power to arrest is abused and used on protesters or is racially biased.

New York City
In New York City, Michael Bloomberg's administration implemented a "Stop and Frisk" policy. Although it was supposed to prevent crime, it actually was not as effective as it was claimed. In 2011, blacks and Latinos made up twenty-four percent of the population of Park Slope, Brooklyn, NY. However, seventy-nine percent of people who were stopped and frisked were black or Latino.

Although a judge struck it down, that verdict was blocked by an appelate court.

A lot of these things were based on measures in the NYPD. In some precincts, such as the 81st, officers were threatened with demotion if they didn't issue a certain number of tickets and arrest a certain number of people. Because of that, you could be arrested for just being on the street.

Civil Disobedience and Protesting
Sometimes, people try to be arrested on purpose by braking what they see as unjust laws. Henry David Thoreau, in his essay Civil Disobedience wrote that

"Unjust laws exist; shall we be content to obey them, or shall we endeavor to amend them, and obey them until we have succeeded, or shall we transgress them at once? Men generally, under such a government as this, think that they ought to wait until they have persuaded the majority to alter them. They think that, if they should resist, the remedy would be worse than the evil. But it is the fault of the government itself that the remedy is worse than the evil. It makes it worse. Why is it not more apt to anticipate and provide for reform? Why does it not cherish its wise minority? Why does it cry and resist before it is hurt? Why does it not encourage its citizens to be on the alert to point out its faults, and do better than it would have them?...."

....Under a government which imprisons any unjustly, the true place for a just man is also a prison. The proper place to-day, the only place which Massachusetts has provided for her freer and less desponding spirits, is in her prisons, to be put out and locked out of the State by her own act, as they have already put themselves out by their principles.

Though Thoreau's ideas may be radical, they have been employed a lot since than in protest of various laws. They worked during the Civil rights movement, although it took a long time and a lot of human rights violations on the part of the Police.

At other times, people show up to attend a protest, without getting arrested, but end up being arrested anyway. The protest may become civil disobedience if the protesters are threatened with arrest if they refuse to leave and then refuse to leave.

Regardless of the context, arresting somebody for protesting is a clear violation of the US Constitution and needs to be stopped. However, it is still extremely common for protestors to be arrested sometimes violantly. The list goes on including people pushing for immigration reform, protesting the Keystone XL Pipeline, and observing the Texas Senate during the debate over abortion.