If they're trying to influence public opinion and there really are a bunch of successes from surveillance programs that can be defeated with consumer-grade encryption, they could find ways to declassify statistics.
I suppose there's the concern that a statement like "we prosecuted 2387 child molesters based on surveillance of unencrypted chat" would cause future offenders to stop using unencrypted chat, but I think that's a red herring. Any reasonably clever criminal would not be sending felonious content unencrypted today.
This is one of those classic problems where if they didn't violate civil rights and something terrible happen the public backlash clamoring for civil rights violations would push them 10x further into outright oppressive practices (i.e. after Sept 11th in the US) than if they'd just broken the rules in the first place.
Just that its easy to imagine being in that role, being privy to some insane shit that barely gets stopped, and never again thinking twice about violating all these rules just to prevent that kind of thing.
Doesn't mean what they're advocating for isn't messed up. But its not hard to think through how they get there.
I find this much more likely than others' explanation that they're evil, that it's just about power, that there is a conspiracy among world leaders, etc.