I don't imagine it was deleted because SDN judged it to be a "troll" thread or in an attempt to either downplay the significance of the event (ignoring blatant racism/Islamophobia) or to avoid a heated internet clash.
1) SDN moderation does a good job of locking down troll threads as soon as the thread, beyond a shadow of a doubt, is revealed to be a troll or nonsense thread.
2) There have been SO many heated clashes on SDN over the years. Some have even happened simultaneously over similar - frequently racial, sexual or otherwise sensitive - topics across different "board" on the forum.
My hypothesis? If the OP's story was true those are some very serious allegations against Einstein and while I am sure that the school would never stand behind such statements, I do not think it is impossible that one of their faculty could harbor such views as those views are not unheard of, generally speaking, though I would certainly hope that none of their esteemed faculty would express themselves in such a crass, ignorant, prejudiced way.
In light of that, I would have done the same. Taken the post down, contacted the OP and told him that he should bring this up with the Einstein administration as soon as possible with whatever evidence he can and further explain that due to the gravity and scope of such allegations a public forum would not be the best place to discuss such matters. If the thread had been about Islamophobia in general, as I'm certain this one will soon become, I doubt the SDN mods would have done anything other than keep an eye on it for civil behavior; however, an individual and in fact a whole institution was accused of what anyone would recognize as gross misconduct and what most academic institutions would be openly disgusted about.
EDIT: Others beat me to it and in less words.