That's not entirely true. ISIS fuels its ground combat force with angry young people recruited via predominately through social media. The social media often consists of video sharing and much of the video is served up from sites like isdarat.tv (now apparently gone) that are/were fronted by CloudFlare. YouTube and Facebook are efficient with their censoring; ISIS needs stable hosting for these videos for maximum efficacy and CF provided them with that.
Gagging these people certainly won't calm them down or stop their communication but it has a real effect on their recruitment, which is directly driving their combat operations in the Middle East and as a second-order effect, the flood of refugees to Europe.
Citation, please. I hugely doubt censorship has clear, measurable effect on recruitment. Among other things it's got to increase resentment of the western world.
Finally, there may be extenuating circumstances of which we aren't aware. Perhaps CloudFlare is granting them use of data in exchange for not cutting cables. When you're that big your presence is indistinguishable from the internet itself.
I cannot readily cite any scholarly papers on this subject; this is from my observations as a frequent reader of /r/syriancivilwar, jihadology.net, and Iraq/Syria-related social media content. The typical pattern is for ISIS to release a video to their propaganda sites and for jihadist social media users to tweet the link.
al-Ḥayāt Media Center (ISIS's media outlet) relies on the ease of distribution via protected (proxied) channels to reach their large audiences. The reach of this content would be more limited if companies like CF wouldn't shield it.
Gagging these people certainly won't calm them down or stop their communication but it has a real effect on their recruitment, which is directly driving their combat operations in the Middle East and as a second-order effect, the flood of refugees to Europe.