Did it? How many people do you think were interested in old school conspiracy theories like Area 51 [1] or the protocols of the elders of zion [2]? Henry ford printed half a million copies of the latter!
>how exactly are Milo Yiannopolous and Paul Joseph Watson "dangerous"?
I think "dangerous" is probably not quite the right word. They're dangerous in the same way that, say, anti-vax views are dangerous [3]. The views themselves won't hurt you or hurt me. They're 'just' opinions. However, at some point, allowing those views on your platform make you complicit in promoting pandemics. You're not directly responsible exactly, but you're involved somehow.
Where the "line" is and how we talk about it isn't clear, but I think there is a line somewhere and I feel pretty comfortable that the people Facebook banned are probably on the other side of it.
[1] https://en.wikipedia.org/wiki/Area_51#UFO_and_other_conspira... [2] https://en.wikipedia.org/wiki/The_Protocols_of_the_Elders_of... [3] https://www.cdc.gov/measles/cases-outbreaks.html
Edit: I should note, lest anyone think this is all "in the past," that people still claim that "the protocols" is an authentically jewish document that describes real Jewish people.