The reason your devices die if you unplug power from the amp is because as soon as you do, the amp stops passing signal. They all work like this and there's no way around it.

As to why you need the amp, only an on site tech can tell you that. There are lots of reasons.

You could be on an end of one tap where signal levels are lower by design. You could have an overly long drop and the amp is required before splitting. You could have more outlets than the levels coming off the drop can support so the amp is needed to boost signal before splitting.

The devices that you're seeing on the splitter and coming into the amp are most likely not filters. Sounds like the tech knew that he was doing. Amp is in the proper location (at drop) and he either EQed or simmed the levels down for each outlet as needed to provide proper, flat response.

It doesn't sound as though you're currently having any issues. (please correct if I'm wrong) so I'm wondering why your ISP is a "deadbeat". From what you've described, the tech did his job. Amp is required and he put devices in place to flatten the response across the band. If your concern is interference from the amp, don't worry about it. Modern coax with proper fittings and tightened connections doesn't allow ingress or egress.

Without a good SLM, no one here can tell you if the amp is required or not. If you're not having problems with your service then there's no reason to futz with it.