Ah, but the 'undesireable elements' round this part of the world tend to resort to portable angle grinders (or even oxy-acetylene cutters in the past) if DCA makes life too difficult, so they will get in if they are really determined - and some of them are really, really determined. And that's not cheap to put right, so this seems the best compromise all round so far, given that multiple locks and keys are more expensive anyway.
After the B******s used the oxy-acetylene cutting gear on the Clayton Adit at Ecton Mine back in the 1970s, the owner gave in and walled up the entrance - end of access for all. (Though it did avoid the risk to local kids of a 900 foot flooded shaft in the middle of the chamber with ankle deep water which you walked across - which was the reason for the lock in the first place.) Now portable angle-grinders are the thing so DCA does the best it can.