It's a power play, and Clemans' disdain for rigid power structures clearly contributed to the friction between him and the SPD administration.
EDIT: the captain's response is not unwarranted at all. Emergency response call centers are a critical public resource and he is ultimately in charge of them.
If Clemans screwed up, he had the potential to have 911 calls be routed to /dev/null. If he succeeded, the department became more efficient. However the 911 system is already working "well enough". Therefore in terms of outside perception any win will be small, and disasters could be huge.
He is not the one who has been give the authority to make this kind of risk decision. The captain is. If Clemans screwed up, the captain would be the one getting fired. Keeping the captain out of the loop is irresponsible. It doesn't matter that it worked. It doesn't matter how good the system is. You need to work with stakeholders, and not behind their backs.
However Clemans seems like the kind of person who can never understand this point.
You are absolutely correct that he had no authority to perform modifications to a critical public service, and that his actions bordered on reckless and negligent and that his response to the captain's accusations was cringe-worthy as well as indicative of the ignorance of his actions, but I think it stands to reason that the captain could have handled the situation more diplomatically.
Granted, I would probably get into a shouting match with Clemans for what he did (and I'm just another developer), but that hardly justifies barring him from entering the premises until further notice.
Clemans' actions can be chalked up to youthful ignorance and exuberance, whereas the captain has no excuse. He is the central source of truth and the ultimate arbiter in the situation, he should know better than to react so aggressively to one of his own staff members.
Touching mission critical things always has risk and those risks have to be mitigated at a political level. If the programmer is this bad at respecting political ownership of responsibility, he has no business touching production this way.
And you want to say the PD could have handled that better? Go into your boss right now and do the same thing. Let us know how he handled that from the unemployment line.
That said - I work for a private firm, not even remotely as crucial as the police dept. cops are trained to deal with all kinds of situations/personalities (my boss is not). The captain could've banned him for a month, instead of permanently. After both of them have cooled down, he could've sat him down and explained that shouting isn't going to help. He could've given him warnings etc. Instead, he seems to have gone on a power trip. Clemans is not some destructive guy, or some criminal. He is just a frustrated over enthusiastic young man. The real losers in this are the people of Seattle.
I hope some other police dept hires him and also he learns to control himself better.