Im not sure that I missed the point, I think that we are looking at the same point from different perspectives.
Of course airlines do not surcharge passengers for night flights because a large bulk of their business is night flights, the pax have all bought a ticket and crew have been rostered. No problems here.
FBO's that reside on 24/7 airfields generally have nil business between about 2200-0600 and therefore do not roster staff as it is not cost effective. The airport make handling mandatory, not the FBO. I promise you that FBO's do not want the problem of staffing during the night. The HSE now insist on nobody working alone and all that nonsense so you need a minimum of a couple of staff. They have to be paid overtime as they generally wont work for free and have not been rostered. On top of the overtime you have to actually ask someone to be on call and not drink etc so standby payments kick in.
All I am saying is that from a business perspective an out of hours fee at an FBO can actually be explained. I agree that some FBO's charge nonsensical fee's for all manner of things and that is wrong. Personally, I have always published my fee's and charges each year via website, sent them out to each operator by way of letter and emailed them also to regular customers. Nothing hidden, all very transparent. Did you enquire about the fees before you went to any of the places commented on above? Did you look at their Acu-Kwik entry or similar?
Some of the comments on here are valid and some are not. My view is simply that if I provide a good service then people are generally happy to pay the bill. If I dont then they should not be asked to pay and the FBO deserves all the criticism in the world.