The only similarity I see is that Novel made an agreement with Microsoft and Yahoo made an agreement with Microsoft. The agreements are about completely different things.
Yahoo's agreement was to use Microsoft's technology as implemented by Microsoft and Novel's is that Microsoft will not assert their patents against Novel for implementing a competing product. (Although I don't know how useful Novel's agreement is in regard to Mono given that Microsoft later promised not to assert patents against anyone for creating or using an alternate .NET implementation back in July.)
So, on one hand you have Yahoo dropping out of search (not competing) and the other Mono continuing doing what they've been doing (providing an alternate competing product).
Mono is not really a competitor to .NET and so far it doesn't seem it ever will... It will always just play catch up or be a C# for open source devs. It's like saying that Wine is a competitor to Windows 7. It isn't.
Anyway my point is that ubuntu is not a distro that stays away from controversial or MS techs or techs based on MS ideas (Call that whatever you want) just because of some ideology or MS hate.
... and Mono for sure has many haters among community (far more then samba don't you agree ? ) So I think it's pretty valid to say that if ubuntu has nothing against mono it's no surprise they also have nothing against using bing based search.
BTW Honestly I don't use / care about mono or what the default search in ubuntu is. I was just surprised at how people are surprised with ubuntu decision ... That's all.