Wow, horrifying. I want to give them the benefit of the doubt for such a tradgedy and the emotional stress and grief it causes; but this is illogical, dangerous and counterproductive. Terrorism seeks to change our way of life and sueing a company for providing a platform for[0] free speech (which those companies actually already curtail) will not only cause problems to this already slippery slope, but simply displace terrorists to less visible places.
I hope this is not greed, because these companies are about as well funded as they are unconnected/innocent in this incident.
[0] added "provide a platform for" free speech for clarity.
Also,
I am aware these platforms moderate, Facebook just committed to hiring 1400ish people to mod the site.