That aside, I cannot see any rational reason for the existence of unrequested 'handling' charges - being escorted or specially dressed around aircraft is unnecessary, and there are better, zero cost ways of achieving the same customs security.
IMHO it happens because the handling agents are working hard to make money, and a bit of subterfuge is regarded as OK - as in much of aviation as a whole.
For example, take this common medium-size airport scenario: there are a lot of based firms which have a special landing fee deal. There are also aeroclubs with similar (even lower) deals. There are also handling companies.........
Any GA pilot visiting one of the above firms, or the clubs, avoids paying handling charges.
But the handlers don't like that, so they use various tricks to get money off you.
Last time I was at one such local-ish airport, the 'mandatory handler' quizzed me on exactly what 'maintenance' work I was having done. He obviously smelt a common local wheeze, where people arrange to park at one of the firms when meeting up with somebody... On another occassion, he said I need to go via him even if going in for maintenance (which was flat wrong).
Handlers often drive up to you after landing, to trap you into an implicit use of their service. Only the locals will know enough to resist, but the locals are not going to get approached
The people who we have to blame for these scams are essentially the airport managers who allow these practices to continue right under their nose, despite a torrent of complaints. In some cases the manager is getting a backhander from the handler, facilitated by e.g. there being "no available parking" unless the handler is used.
But the other party to blame are the bizjet owners who pay anything (short of a grand) without batting an eyelid. They have spent £10k flying there so £500 handling is immaterial.
Put all this together and it's no wonder there are airports where you land and an army of parasites descend on you. Incentives always work the same way on people, whether it is an airport, or a bank
I have never flown to say India but have heard plenty of reports of several officials lining up in a line and you have to get a roll of US$ out and pay them off one after the other. Then they charge you duty on the unused fuel in your tanks. A load of crooks.
This is one reason why I always advise pilots to contact the destination airport and check out the 'deal' before flying there. Anyway, one needs to get stuff like Customs PNR, avgas, opening hours, etc, out of the horses's mouth. No good reading the AIP; most of the southern European ones are full of crap.
Europe does not have the corporate transparency of the USA and never will have, because airports are mostly not an integral feature of the economy and exist in a sort of twilight zone, where the manager gets more or less anything past the usually clue-less local authority, under the banner of 'safety'.