I accidentally deleted a post by ehoque about the Bing TOS and various parts that DDG might be violating. My view is that Microsoft (and Yahoo and others) know who we are and what we're doing and they don't have a problem with it.
I think people get too scared off with TOS. Lawyers write that to prevent abuse but that doesn't mean the operational people really care about each and every point in every situation.
DDG seems to be the model use for these APIs, and I get that sense when talking with them.
As for particulars, I have attribution on the FAQ page. I do differentiate other sources, e.g. the 0-click box.
I do remove spam, but I don't think this violates the spirit of that requirement either. They are concerned with people mining their data, i.e. querying lots of stuff and not displaying it ever to end users.