Well, this guy is the mayor, and he has a record, and these guys knew what sort of person they were dealing with, and they didn't have media with them so....I'm not sure what that has to do with an unrelated incident in Indiana.
If you have an arrest warrant for a person you know to be armed and unstable, why serve it in the most dangerous way possible? Why give him the benefit of a reinforced door, and the protections that his home afford? He has a job, and you know where he works.
This is especially so, if you have a search warrant that is going to get you into his house once you pick him up anyway.
Like Denny said upthread...