The answer is that most democracies have various fundamental protections against spying on their own citizens (including the USA), because citizens want that. But governments still want to spy on their own citizens anyway, because terrorism and think-of-the-children and the war on drugs and geopolitics and general control of 1%ers over 99%ers. So it's not even really about non-Americans.
So what to do? How to get around that, in a way that citizens don't mind? Why, you set up a secretive Five Eyes agreement (also the other "Eyes"). UK spies on USA citizens, USA spies on UK citizens, the USA shares that gathered intelligence with UK, and UK shares their intelligence with USA.
Voila! Now each government can spy on all its citizens via trusted foreign proxies. It's legal, and the citizenry is not informed enough to care.
Anyway, that is the answer to your question. 1%ers want it, and citizens are not informed enough to care.