So this argument is complete crap when it comes to governments but totally okay when it comes to spouses?

How about a series of emails joking & griping about your wife's habits to your best friend? Completely harmless but not exactly the kind of thing that you'd want them reading.

Our digital selves trail back in time farther and farther these days; would you want your wife reading emails between you and an ex from before you met her?

There's a deeply nested thread, right above, of people arguing against that idea. You just need to replace 'wife' with 'NSA'.

I just point it out because I find it's funny.

I agree trust and honesty is part of a healthy relationship.