Earlier quoted context omitted.
Why would they want to train on random garbage proprietary emails? If their models ever spit out obviously confidential information belonging to their paying customers they'll lose those paying customers to their competitors - and probably face significant legal costs as well. Your random confidential corporate email really isn't that valuable for training. I'd argue it's more like toxic waste that should be avoided…
Your opinion seems a little unimaginative. To me, since email is the primary work output of millions of Americans, including all of its leaders, there is a lot of opportunity there.
Valuable enough to take on that moral, legal and financial risk?