Yes they're trained; they're trained to prevent situations like this and diffuse them if they happen. He had already established Gates's identity and that this was his residence.

All he had to do was say "Someone thought that it might have been a break-in because of the stuck door. I apologize for taking up your time but I need to get your version of events for the case file. Is now a convenient time for me to do that or would you like me to give you my card so you can come down to the station at a more convenient time?"

But he didn't do that because he was pissed off that Gates wasn't kowtowing to him and he wanted to teach him a lesson. Whether it was a racist reaction or not it was still the wrong reaction and an abuse of power.