It’s policy. Most agencies have that policy, not because of liability, but because they don’t know who might be the person doing the “upgrades”. An upgraded trigger done properly by someone that knows what they’re doing, is an improvement. One done by someone that doesn’t know what they’re doing can make the weapon less reliable.
Basically, same reason police cars have to be worked on at the agency garage.
My SIG P229 duty gun does have a Short Reset Trigger, which a modification developed by SIG. Not all our guns have it, but some of us have requested it and they’re installed by agency armorers......of which I am one.
But at the end of the day it's still a matter of liability. The department knows that if they let everyone do whatever they wanted to their duty guns, a single mistake could cost them a fortune.
And the reason LEOs have their own armorers is so that they can bring records into court showing that the duty guns were only modified in accordance with department policy. As LEO you can always have someone with all kinds of credentials come in and prove in court that the gun was in it's unmodified condition and was well maintained and functioning properly.
Non-LEO don't have that luxury if we go to court. It's up to us to refute any charges that the prosecutor may make to try and break down a self defense claim. That's why I think it's important to have the manufacturer on your side.