I suspect MS is trying to keep Bing as a separate entity. Perhaps to avoid feeling like the big monolith threatening to over compete.

Alternatively it might be just trying to make Bing feel like a new brand... who knows.

If you look at bing.com the only overly obvious reference to MS is the MSN link at the top left, and the microsoft copyright.

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.

The Bing marketing people may not be interested in promoting the fact that it's a Microsoft product - their aim is only to get more people using Bing.

In their shoes, if I had to chose between promoting the fact it's a Microsoft product and promoting it's a search engine, I would go with #2. The fact it's microsofts is irrelevant to the consumer.

People hate Microsoft. They associate it with a necessary evil which breaks a lot. Maybe that has something to do with it?

One reason might be that he was trying to wring another 2500 out of them by saying Bing instead.