"If you are not obstractring the police work or endangering anyone, why should one be arrested for taking photographs in public even if they are police officers. I thought they exist to assist and protect lawfull citizens not to harm them if they are not over stepping the lines."

Well, I actually agree but...

This would have to do with the perspective of the police vs the perspective of the photographer. The police officer's view of what is obstructing or endangering is what counts at the scene. He is in charge. You might be proven correct when the case goes before the court system but you risk being arrested in the interim.

Also, police do not exist to assist and protect, despite the romantic ideal and the slogans on the doors of police cars all over the place. They are there to uphold the law. If it's a bad law, a crappy law, a law that stinks so bad they have to hold their noses to uphold it and they hate doing it--it's what they are supposed to do. If they can assist and protect in the process, it's a side benefit.