I'm not sure I agree with this. Surely the solution is not to bother with the completely pointless createAdmin and createCustomer functions in the first place a

I'm not sure I agree with this. Surely the solution is not to bother with the completely pointless createAdmin and createCustomer functions in the first place and to just call createUser directly where it is needed with the correct parameter. Otherwise you're just repeating yourself. Most of the time, DRY is better.