Leaks have shown that with the 5 eyes 'one hand washes another' and probably spying on US citizens on behest of the US.
The intelligence community has their hands tied when it comes to their own citizens due to strong constitutional protections, so in order to get what they desire, they operate quasi-extrajudicially (i.e. exploit a loophole) where they just get a friendly foreign power to do the spying and then trade intelligence for intelligence on their own citizens. I'm pretty sure this kind of scheme is behind GP's post and such an arrangement is not limited to members of the five eyes.