Game maker locks you into a set frame rate at the beginning of a project, and you can't change it later. Also, lots of actions/animations (well, pretty much everything) are based on that frame rate, so it's easier to work with at 30 than 60.
I definitely agree, I'm just trying to explain the technical and time-based constraints behind it. And, uh, let you know that it would take a full-on port to get it running at 16.67ms frame-times.
Also, as I believe others have mentioned, it helps to use a frame limiting tool like RTSS.