I don't think Microsoft cares
that much, but I'm sure they view it for what it is: illegal, unauthorized usage of their intellectual property. (I'll hold off on my personal views of 'intellectual property' and whatnot; I'm not a lawyer, and those opinions are hardly relevant to the reality here.)
Still, even if Microsoft doesn't care, Wine Project and ReactOS care. My employer might care. There are unfortunately legal implications that exist, and if people want to play by the books they have to at least do the basic due diligence.
If you genuinely just don't care, you can lie to them and claim you never saw the original code, and attempt to cloak any code theft to make it indistinguishable from clean-room reverse engineering. Nothing stops you from doing that. Hell, the trouble is that it really can't be proven for sure. Even holding the positions I do on forums wouldn't prove my own innocence in such a situation, though maybe it helps build a case.