I think that was a bug. It was happening to me in mostly i TB beta, but it stopped with the last update. So it will make it to release, but not sure when.
@jeffthomas1965, that removes an account, not disables it. And the 'Delete' button is already there, as you learned https://connect.mozilla.org/t5/discussions/thunderbird-account-settings-unable-to-delete-an-account/td-p/127002.
