They do get to work on someone else's server, they do not get a separate account on that server. There client would be not happy to have them mess around with the environment.
Accounts are basically free. Not having accounts; that's expensive.
Let's assume they need access to the full service account environment for the work, which means they need to login or run commands as the service account.
This is a bit outside my domain, so this is a genuine question. I've worked on single user and embedded systems where this isn't possible, so I find the "unprofessional" statement very naive.