Censorship isn't limited to government even though that's the first example that rushes to mind. Any organization or person (i.e. "self-censored") can censor themselves or others. So yes, YouTube is most definitely censoring others who advertisers might object to -- dangerous considering YouTube's position as the new town square.

censor verb censored; censoring\ ˈsen(t)- sə- riŋ , ˈsen(t)s- riŋ \

Definition of censor (Entry 2 of 2)

transitive verb : to examine in order to suppress (see suppress sense 2) or delete anything considered objectionable censor the news also : to suppress or delete as objectionable censor out indecent passages