Could you help me understand what assumptions about the access method you have in place that make this seem unprofessional?

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.