You're right that the U.S. government has been caught with its proverbial pants down, but the sort of defendant who might hire a mom-and-pop legal shop isn't likely to be on the NSA's radar. Also, consider the risks if the government were exposed as having gone through attorney-client privileged information. The blowback from that would be nightmarish. Most any judge would dismiss the defendant and hold the government's attorneys personally in contempt. Or worse.

Also, it's worth noting that lawyers, more so than anybody else, have finely tuned antenna for what should be in writing and what should be ephemeral. For most lawyers I've dealt with, emails are for arranging phone calls. Phone calls, then, are assumed to be unrecorded. If it came out that the government were recording the content, not the metadata, of entirely domestic phone calls (many subject to attorney-client privilege) and was using that data without having the appropriate warrants... it wouldn't be pretty in court.

Not to say that such things haven't been going on. With all the Snowden revelations, it's hard to say when it's all going to end.