Exactly that: you open-sourced it so it's your donation to the world. Don't be sour if someone copies your idea or makes a ton of money from it.

And what would be the benefit if Microsoft gave you credit for it? Most likely their lawyers would reject it since you may then be able to sue them for...I don't know what, but money in any case.

It's a very Microsoft-thing to do to copy someone else's idea and improve on it (C#, RDP, Excel). If you release something as open-source you have to ask yourself if your doing it out of altruism or for money? In case of the latter you have to plan accordingly, by patenting or with restrictive licensing.