Referees can do what they want, they don't get paid to train they do all that on their own time like the meetings etc. Though if you get the higher levels then that is paid for (Level refereeing badge courses). Then we get reimbursement for our games and how much depends on the level they do, I think from what I have heard, Sunday league pays more than a referee that does a capital football game by themselves pays.
No contract will be signed stopping them from doing another competition.