As has been said BOH is not the only regional airport to charge for drop-off; many do so.
I can remember the time a generation ago when many smaller UK regional airports had few scheduled flights and where they were operated the fares were very high compared with today's low-cost airlines.
In those days the smaller airports made much of their money through airlines paying the full charges for the services they provided. Nowadays low cost airlines demand next to nothing charges in return for which they will bring a substantial footfall of passengers through an airport.
The airport therefore has to make its money from ancillary sources such as car parking (including drop off), retail outlets, trolley charges, fast track security etc.
If an airline was to put a pound or two on a ticket I doubt that many customers would complain, if they even noticed. Put the same pound or two on a charge for drop-off parking and complaints come from everywhere.
My local airport is BRS and I can visit, say, GLA or EDi for around £50 there and back with a decent frequency too. Contrast that to 25 years ago when BRS had hardly any Scottish flights and those that did operate cost £150-£200 return at 1990s levels. I know which of these two situations I prefer and if a couple of quid drop-off fee is the price to pay it's really nothing in the overall context.