Questionable?
Questionable? Not really. Councillors are well aware that certain material is confidential and should not be circulated.. it doesn't mean that they don't do it though, and often the release of this confidential information could be defended as being in the public interest.
Councillors are not exempt from RIPA, so if the council has an appropriate policy and procedures then a councillor's email can be intercepted without notice.
And it's not a political issue either, if the councillor was breaching confidentiality then it is a disciplinary one. There is a quite separate issue between Cllr Abrams and his local party.
But.. only someone really stupid would leak sensitive information using the council's email system anyway. However, a number of councillors are busted each year for looking at porn on their council laptops which is even more stupid.
It does seem to be all rather harsh on Cllr Abrams, but assuming that El Reg is 100% accurate in the facts then it is likely that the council has acted within its authority, internal Labour party squabbling is up to them.