They aren't the only ones who do this: https://www.freebsd.org/cgi/man.cgi?accf_http . All sorts of things are kernel-accelerated on modern operating systems, including lots of network operations. From that perspective, this is just one more and it could potentially have a huge benefit (like the same page served count on fewer hardware) for customers who need it.

Obviously it's critically important that MS get this right if they're going to offer it at all, but that's pretty much a tautology when talking about kernels and core OS functionality. Judge them on a bad implementation, not on any inherent badness in the idea.