I have set by accident the default program to open ".sc2map" files to be SCMDraft...
And now I somehow can't revert it back to StarCraft II Editor.

I right-click on a sc2map file, select "Open With", then "Browse", find the StarCraft II Editor program, select it, and press "Ok". Somehow, it just doesn't want to appear in the list of programs I can select from there on.

I'm using Windows 7.

When selecting the SC2 editor, are you checking the checkmark that says "use as default program to open this type of file"?

If that's not the culprit try the instructions here (first try the response marked with a green "Answer")
http://answers.microsoft.com/en-us/windows/forum/windows_7-files/problems-with-file-association-in-windows-7-64-bit/8a84fcec-22df-4942-8e35-d98dbe96e327

Worked perfectly.
Thank you very much.