There were rules in QRs. I think 'men' could request permission of their CO for an external part-time job. For instance they would have to convince their CO that they could do 4 hours in a Pizza Parlour after work without affecting their day job and that there would be no conflicting demands.
For officers I don't think it was an option.
Now consider a pilot who wanted to fly for a few days while on leave. At first glance there is no conflict. However leave is supposed to be to ensure the individual has adequate R&R.
Suppose the commercial flying involved a large chunck of flying hours then he could be out of permitted flying hours just when the military required him to be fit and rested.