The same could be said of the CLR, and there have been efforts like Mono to replicate it independently of Microsoft. Nevertheless, in practice Microsoft control the .NET platform and the CLR. Like any other platform, they can control what underlying features of the OS or hardware it does or doesn't support, and they can give their tooling an advantage over free, open source tooling.
For example, what if they add a new set of op codes to the next release? Don't their tools arrive in the marketplace with full support of the new op codes while the free, open source tools must toil away to replicate them?
We can quibble about how much advantage their tools have in the marketplace, but unlike the late Sun Microsystems they are very clear about being in the business of making money from tools and commoditizing operating systems, browsers, and hardware to support that business model.
Except Microsoft doesn't provide fully functional implementations of the CLR for a reasonable set of platforms. Microsoft intentionally crippled the CLR on non-Windows OSes, and continues to do so.
For example, what if they add a new set of op codes to the next release? Don't their tools arrive in the marketplace with full support of the new op codes while the free, open source tools must toil away to replicate them?
We can quibble about how much advantage their tools have in the marketplace, but unlike the late Sun Microsystems they are very clear about being in the business of making money from tools and commoditizing operating systems, browsers, and hardware to support that business model.