Democracy fundamentally relies on freedom of expression: being able to discuss problems, to organize protests, to resolve issues (like corrupt politicians).

If you conduct mass surveillance (as does the NSA, the brits and apparently many others too), then people will refrain from expressing themselves freely, out of fear of blowback. Problems will not be discussed anymore, solutions will not be proposed and corruption will flourish. That's the end of democracy.

Usually, schools teach these basics.

Actually, not really. Germany was awarded the '36 Games in 1931 which is before the Nazis came to power. And even in '36 it wouldn't have been so obvious exactly how bad the Nazis were going to turn out to be. Much easier to say in hindsight.