They are quite probably reading it, and I think I read something about a buffer of a few days that they kept things for. But they're probably not keeping all of everyone's communications in case they want to refer to them in five years time. Whereas on GMail they can dig back through years of old e-mails as well.

At the very least, it adds a small per-person cost to their surveillance, because they have to store copies of e-mails themselves rather than getting Google to do it.