GCHQ employs great mathematicians when they're young. It gives them a pleasant working environment. There's probably travel to US to work with American colleagues, and similar travel from US to work with UK colleagues.

There are plenty of genuine adversaries to use in recruitment.

And then, when you're doing the job, you're just cracking crypto or finding exploits, you're not spying on your neighbours.

There's a few fun bits of propaganda - GCHQ HACKS TERRORIST WEBSITE, REPLACES BOMB INSTRUCTIONS WITH CAKE RECIPES - for example.

So, you have these really freakin' smart people working on these interesting problems. They can learn from the rest of the Internet. They just can't share their learning back.

And then you have some managers somewhere reading the law, and coming up with some interesting non-conventional interpretation, and making use of all these interesting exploits and broken crypto systems.

Parliamentary oversight fails for some reason.