I actually got a chance to meet the creator of SDN (a really nice doc in So Cal) and he said that they weren't going to have any adverts but then settled on google because google does all the leg work and upkeep on their adverts. Also, he said that they thought google would be useful to students at least. Apparently they had a lot of "biased" companies trying to buy ad space and the creators didn't want really misleading ads. The site is non-profit so they needed at least one sponser to keep it working.
Oh and he didn't say anything about donations except that they aren't always steady.