Why assume that by coming to deal with someone sitting in the cab of a train they're suddenly going to be missing out dealing with youths putting breeze blocks on the line?
If BTP are so short of staff that they're having to ignore certain problems, they should be fighting for more money and resources - not just giving up on doing their job.
Of course, we seem to have a mix of opinions as to how under-resourced they are, compared to the reports from staff about how they'll just refuse to come out to many things - thus undermining staff. If you're a driver, you might want them to come out to deal with the breeze blocks, but what about your colleagues being given the grief on the other side of the door on your train?
RPIs will go after people in first class without a FC ticket, so I find it hard to believe that anyone should get the impression that buying a key and upgrading to the rear (or middle) cab is acceptable. If word got around, you'd soon find that there will end up with a few people trying it - and as they fill up the cab, it will increase the chances of something being pressed that shouldn't.