What astonishes me is not that it's happening, but the sheer scale of the thing. I would find odd my mail being of interest, but why in hell would anyone be interested on what my mom exchanges with her friends?
To ensure complete information collection, it would not be smart of any spying agency to only target "suspicious" email accounts, when a suspect could use their own mother's email to communicate to other suspects' mother's email.
Much like airport security does random searches instead of targeted searches (to only people with Middle Eastern features, for instance), randomizing probably helps. It helps even more to gather information from ALL email accounts.