Cross-compatibility makes a lot of sense of they perceive a lot of future risk to their value proposition that one technology or product will sell another one. If they don't believe Windows Server will be a compelling enough technology to push adoption of .NET, then why risk the decade of development on it by tying it around the neck of IIS? I see it as an admission that the various product offerings need to float on their own merits, not just because 'its the option that runs on Windows'. There's enough alternatives now to make that kind of assumption a relic of the '90s and 00's.