When "normal" citizens can find instructions on how to create IED's and pipe bombs on the surface internet, something is very very wrong. This is what causes naive easily-influenced youth to radicalize. They do not even have to seek it out on the deep web, their social feeds are filled with such extremist content.

Then when the police confiscates their computer and finds Inspire magazine on there, they are branded a terrorist and face jail. While there is a difference between curious rebellious youth and hardcore jihadists, it's very murky when there is such easy access to real damaging stuff.

Driving them underground would make anyone seeking them out a solid suspect. Right now innocent bystanders are confronted with their propaganda and may give in (whereas normally they never would).

I know these are American companies, but they are used world-wide. The bill of rights and American standards of free speech do not have to apply everywhere though. Europe faces the consequences of this unbridled free speech. You want free (hate) speech? Fine. Create your own website and host your own content. Don't (ab)use these big companies who hide behind our-algo's-could-not-find-it and do not take their responsibility, because they have so many users they can never police them all.

Real terrorists use both the surface web (for recruitment and propaganda) and the underground web (for planning attacks). No smart terrorist uses the surface web for planning. The damage of surface web propaganda far exceeds the damage of having no intel on surface web propaganda.