I have no problem with what they do except the final part should be having a copper at the point where he gets rumbled - then hand over all the evidence and haul him off to the cop shop for at least a bit of questioning. At least then it can be taken further or dropped. I could see that working with a couple of dedicated coppers overseeing these "private investigators"
I think part of the issue is that these private investigators have arisen because of a lack of police officers. I often defend the police on here but even I'm struggling to do that with some things at the moment. Literally the only time I have seen a traffic officer near me in the last year was when they changed a dual lane stretch of road down from a 40 to a 30. He was sat there for 3 days with a speed gun. People take the p and say they're only interested in going after easy crimes that generate income and frankly it's true.
I've been reporting some tossers on motorbikes riding on paths and through a kids playground for 2 years now and the police haven't come out once. It's the same guys and I've even given the address of one of them. Nothing because it's too much work for them. Just like trying to catch a paedo.