At the end of every Bing TV add (at least on US Television), the spoken tag line is "Bing, the decision engine from Microsoft" (combined with the Microsoft logo in the corner), so I don't think the advertisers are shying away from the affiliation.
They aren't trying to hide it, but it does seem they want the name to sort of stand on it's own. For starters, they probably realize that vernacular allows for you to 'google it' or 'bing it' or 'yahoo that', but not to 'microsoft that'. If anything, 'microsofting' something probably involves putting it out of business with predatory business tactics.
Alternatively, it could be a judgement call on MS's part based on their knowledge of the Colbert audience.
