I actually had a very similar experience happen to me a couple of years ago when I moved into my new home.
I decided to take a walk in my new neighborhood. So I just grabbed a key and went out the door.
Well, the key I took with me didn't actually work on the front door, and I didn't have the garage door opener setup yet. So, I had to jimmy a window to get into the house.
I had just gotten a home security system (that morning it was installed), and the long story short is that I tripped the alarm, but because of some issue with using VoIP and the system (had to get the motherboard replaced in the control unit afterwards), I wasn't able to cancel the alarm or speak with the customer service people.
Additionally,one of the new neighbors called the police to report suspicious activity.
So, I got a visit from the police.
When they got to the door, I let them in, let them look around, and was able to produce the home purchase papers as well as two forms of photo ID. I explained to them the situation (perfect storm of new home ownership & Murphy's Law), they laughed, and said have a good night. I thanked them for their quick response, and wished them a good and safe night, as well.
End of story.
Had I followed them out the door yelling about getting a visit, I have no doubt I probably would have gotten arrested for disorderly conduct. I would have deserved it, too.
The photo listed on Yahoo says a lot, I think. First, there is an African-American officer standing right there, so if you're going to cry racism, you're implying that he's a "sell-out". Second, Gates is either yelling, or laughing hysterically---given that his mouth is opened rather wide. Seeing how he has not had anything particularly funny to say about the situation, I'm going to assume he was yelling (hence---disorderly conduct). Finally, the cops don't appear to be 'man-handling' him. They seem to be leading him away carefully. I'd take this to mean that they didn't perceive him as a threat, nor held any sort of personal ill will. They all seem to have a very 'just doing the job' look about them.
I've seen cops that were pissed to the Nth degree. None of these officers in the photo look like that, in my opinion.
I could be wrong, but I am getting the vibe that a professor saw an opportunity for a great book/lecture series, and took it.