Sure, he spoke politely, but a polite robber with a gun is still a robber.
I have to ask, what did he accomplish?
Did he take a dangerous, fully-automatic weapon off the street?
Did he put a dangerous miscreant in jail?
Did he issue a citation? a warning? a sternly worded letter?
Did he do anything to prevent or discourage the same thing from happening again tomorrow?
Did he mistake a sub-$500 .22 long rifle semi-auto carbine with a 16-inch barrel for a $15,000, 9mm sub-machinegun with 9-inch barrel?
Did the two criminal masterminds punch each other on the shoulder and say, "Gee whatta maroon. Hey, let's do it again tomorrow, but next time you carry the .22 and I'll make the call"?