Because you're dealing with sensitive subject matter. Many news sites switch off or minimise advertising on controversial stories. There's no reason Snapchat can't do the same.
It makes no difference one way or the other. The idea is that customer A doesn't want their campaign paired with ads from customer B. I don't care whether they're right or wrong in feeling that way -- they clearly do feel that way. So if you do it anyway, you are absolutely making a clear choice that you favor customer B.
That's my only point. There's no way to abstain and keep the goodwill of everyone.