sorry if this is a dumb question, but: why is it unreasonable for a local admin to have the power to change AppStore preferences? without knowing much about the osx security model, this sounds like not a big deal?
In this case, any credentials work, meaning that if a "guest" user (semi-trusted by the account owner, obviously using the owner's credentials. ) were attempting to change these settings, they could bypass the prompt with a bogus password instead of the alternative which requires the guest to ask the owner to enter their password.