Account preferences can't revert to "unset"

[If you're here because of the "easy" or "beginner-friendly" tags, read all comments and #188.]

On the subscription page under "list based preferences", there are several radio buttons, e.g. for receive own postings.

The description says:

Any options showing as not set inherit at runtime from your global or address specific preferences

This means that there are three states for each button: "yes", "no", and "unset".

However, once set, there is no mechanism to restore the "unset" (inherit from "other tab") state.

I believe this is a bug. E.g., I may want to temporarily override settings for a particular list, but revert it to global preferences later. Suppose I globally disable delivery (vacation), but want e-mail for critical project updates. I enable delivery for that project. When I return, the project is done, so it's no longer "critical". But from then on, I have to separately turn that list on and off because the "unset" state is not reachable from the UI.

Also, radio buttons consume a lot of space for a simple yes/no/unset/ Isn't a 3-state checkbox more appropriate for these items? (click cycles through Y, N, and U(nset/inherit)).