1) The Port hires out its upstairs room for gig use entirely free. There is only one other pub in (reasonably Central) Oxford which does this on the same basis, let alone with anything resembling a decent PA. The money from the door takings goes entirely to the organisers of the night in question. The pub allows these promoters to use the venue in this way purely out of goodwill and for the increase in drinks/snacks takings. This is unusual; normally pub venues retain a close control on bands' income, often setting the door charge themselves and/or offering the band a set fee regardless of how many people attend the night. Other venues charge £200 or more for an evening hire. This may not even include PA equipment, door staff and a soundman. You can see how organisers might be keen to run a night at the Port, then; and perhaps you can also see how the the responsibility for ensuring that the night runs to time is entirely theirs. They are made aware of the limits and if they are unable to stick to them that is unfortunate - but their responsibility.
2) The Port has had trouble with their licence due to complaints from neighbours about noise levels after closing, hence their keenness to close nights on time. You can see how it might be irritating for the staff to have to argue/explain this point every time a band accidentally runs over time. It would certainly stretch my patience. :-)
Small music venues in Oxford are fragile and valuable things. Since Oxford's main central venue for live bands, The Wheatsheaf, is closing down soon we will all have to be even more careful with this precious resource as all the other venues will see an increased demand on their capacity. Hopefully your son's next gig will be at somewhere where the organiser is a bit better at time management - or maybe he can put a night on himself? - and everyone can be happy!