I know this is old, but I just discovered it and couldn't resist:
Never feel the need to talk to the police. You have your fifth amendment right to remain silent.
It is NOT against the law to take pictures in public. It is also not against the law to take pictures of private property.
If you are on private property and told to leave by a representative of the property owners, you must leave. Even if you're in the "right", you are trespassing if you refuse to leave. So if a gas station employee wants you off of the property, you must leave or face arrest. You can, however, take as many pictures you want from the public sidewalk. Take a video of the employee freaking out if you want, too.
When I was working as an attorney we'd have people try to intimidate my boys all the time. I'd send them somewhere to get pictures and they'd be threatened to have the police called on them, and some thugs would threaten violence (keep in mind, it's against the law to threaten bodily harm. In the state of California, "Assault" is the threat of violence. In addition, most municipalities have "disorderly conduct" statutes that cover "fighting words".)
So in conclusion, if this ever happens again just leave. If possible, record the employee telling you to leave. Move to the sidewalk and document the incident. Report it to the company and provide the pictures or video. If the police harass you ask if you're being detained. If you're not leave. Don't waste your time. If you are being detained, ask on what grounds. If you don't want to talk to them, tell them you're exercising your right to remain silent.