I do not think that matters one bit. It’s pretty clear to me that morally, communication someone wrote with the expectation of privacy should not be made public unless there is evidence for criminal wrongdoing. You do not get to change the rules after the fact.
If you want all e-mails researchers write to be public, the researchers have to be informed about that beforehand.
If you ask me to pay for your research, how can you claim that I'm not entitled to see the communications you've done in the process of that research?
I paid for the email server (or for its usage), for your computer and its software, and for your time. I've got every right to see how it's used.
In particular, with publicly-funded research, where we've got an extra step (or several) between the voters and those working on their behalf, the public has a right to know in order to tell their representatives whether they want the government to continue spending their money this way.
UPDATE:
I do not believe that those writing the emails had any legitimate expectation of privacy to begin with. It seems that employees don't generally have a right to privacy in their work emails. For example:
"Privacy" means more than one thing in this context.
Mann did not have (and shouldn't have expected) a right to privacy over his employer, the university. If they wanted to read his emails then they could.
But he might have had (and expected) a right to privacy over the general public, or more specifically organizations like the American Tradition Institute.
You can only get from "the university were entitled to read his email" to "the ATI were entitled to read his mail" by assuming that the ATI had that right over the university -- and that's pretty much exactly what the court case was about, which has just been decided in favour of Mann and the University.
So, yeah, support of FOIA laws is doubtless a valid business purpose, but support of FOIA laws did not require the university to hand over Mann's emails. And this has nothing to do with the question of the university's right to read Mann's email.
You seem to be arguing that making e-mails public in the future would be good. Maybe. But making e-mails written in the past with the expectation of privacy public? That seems ill-advised to me.
Update response: The employer is the university, certainly not the public. (I’m also not sure why you are so quick to dismiss the importance of private communication channels. To me it seems important that people who work together can talk together privately, without the public knowing.)
This. Not to mention, if I knew my emails on my University account would be subject to public scrutiny, I'd just use another email account for all of my dealings.
If you want all e-mails researchers write to be public, the researchers have to be informed about that beforehand.