I challenge you to find a law granting something called "code rights".
There's something called copyright, but that would only apply if
another server were distributing a copy or derivative of Microsoft's
work. It wouldn't stop people from making compatible engines or making
their servers compatible with Microsoft's engine.
There are also restrictions on the license which might get in people's
way if they tried to implement a compatible feature. For example the
license may prohibit using the engine for purposes of reverse
engineering it. Still there is enough documentation that that's
likely to not be a problem. And if Microsoft tried enforcing license
restrictions like that they would possibly be invalidated by the courts.
Microsoft does not, to my knowledge have a patent on any process ASPs
implement. And any blocking patent would be laughably invalid because
of prior art.
Please don't offer legal advice based on imagined rights and laws.