[KingMob]

You're missing a key point in how the spying was actually done: because the Five Eyes nations have tighter controls on domestic spying, they relied on each other to spy on each other's citizens and share the data through reciprocity. Plus, the NSA used the excuse of a single foreign party on a conversation to hoover up all communications just in case. This severely blurs the lines between domestic and foreign spying, and it would be difficult to reveal the details of domestic spying without also revealing how the GCHQ spies on Americans and the NSA spies on the British.

Also consider that Snowden did British citizens a favor by revealing how the GCHQ was ultimately spying on them, too.