I think that if Microsoft makes .NET attractive to a large number of developers regardless of their preferred OS they have a headstart and constant edge on the competition in terms of services that mesh well with .NET. This increases the appeal of Azure and its services. It might be a good shortcut since they're competing with established players who have the early edge. Stealing away a lot of customers from the competition in the cloud space is going to take more than competitive pricing since you're fighting familiarity/experience, first-to-market advantages and a lack of 1:1 features on some services.

The more people they bring I to the .net community the more opportunities they have to sell tons of azure services. Doesn't hurt them to be more open because existing .net shops are still going to pay for msdn, etc.

The same way Office on Mac OS/iOS brings MS Money... services. Office365, Azure, etc.

They use .NET and build their tools and services using it. .NET gets better so does their associated services built using it.