There's some inspection in practically every transport hub when I travel. While I traveled internationally the most strict was in Germany. I really don't mind a lot - there are good reasons to be careful - but they had guards in full military gear and with automatic weapons (usually it's handguns and plain uniforms) which looked pretty intimidating, and that was the only time I had to actually turn on my laptop to show it's a working laptop and not some kind of trick. Maybe showing how strict and tough they are was the point. The worst inspectors I had were in London. They were exceedingly slow and had very unpleasant manners. Maybe just my luck. Never had any real problems though - worst thing they got a look on my underpants and power connectors, and sent me on my way.
There are many more armed services in the US than police/military. Dept. of Education has a SWAT team. Amtrak has armed force. US Park service has one. Really, there are so many of them.
Those are all examples of police agencies with police powers. This is in contrast to TSA officers screening your luggage, who have no police powers and are not police.
To add another example to your list -- the TSA also has their own police (e.g. Federal Air Marshal Service), but they don't work the line screening your baggage.
They are not police either. They are just armed men at government service. TSA inspectors in particular are not armed, but that's immaterial - there are other armed government workers around that could be used if needed. Police is only a tiny part of government's armed forces. It is most visible because everybody seen the police drive around, but most people never saw DeptEd SWAT team, but that doesn't change the fact a lot of those exist and are around, just a bit out of sight.
