I guess they are using the same justification as GCHQ; they use tools to scan everything and a human doesn't actually read ("rifle through") the majority of material.
Is it still an invasion of privacy if a machine reads my emails? Google read my emails to check for spam.
To me, the main difference is that you know about Google's automatic parsing of your emails upfront and it can therefore be a factor in your subscribing/unsubscribing decision.
Is it still an invasion of privacy if a machine reads my emails? Google read my emails to check for spam.