"Dangerous" is one of these words like "change" - on their own they don't give any indication which way they are pointing.

Anyone who says they are in favour of change, until they know what the change might be, is dangerously crazy, and the same goes for "danger".

But in the context of Pete Seeger, what I take it he means is something like "these are songs which are saying things that need to be said. And some people are going to see them as dangerous songs, and rightly so - because it could become harder for them to hold on to their unfair advantages, if other people really listen to these songs."