Quite often administrators only have a limited set of tools available to them, not full access, and are subject to at the least internal logging. Just think of the administrators of banks...

In the case of reddit, I would expect "regular" administrators to have a limited set of tools available to them. Sure, the developers could easily add such a tool, I just wouldn't expect them to do so. Spez is a special case since he's the CEO, so he probably has access to everything. The difference there is one of broken trust - you just don't expect the CEO to cripple his product with such a blatant violation of trust.

No matter how much people on reddit complain about the admins, they still have some implicit trust in them. They trust the admins to not be snooping on their private messages. They trust that the admins aren't gaslighting them by showing them different pages(e.g. imagine users of The_Donald not being able to see controversial posts in other subs to stop them from "brigading"). And they certainly trust the admins to not usurp their identity by silently editing their posts in a way that makes them come off differently.