I think, maybe, one of the issues about this spy scandal is what was reported to be happening.
What I recall hearing about, was that quite a low ranking German official was just handing classified documents to their CIA contact.
So it's might be more of a "If you're going to spy on us and get caught, at least try to make it look like it took some effort" thing. So that if the CIA are caught spying, they're caught doing something that makes it look like they had to expend a bit of effort. Also, make it look like the German agencies are competent, and so on.
The whole thing about getting caught spying on your allies, is that it makes both of you look bad, in front of everyone.