Why would it make sense to not be able to video in 30fps in a 720 resolution, and have to video in a higher resolution to get 30fps?
720 is typically captured at 60fps, not always, it's not required (different video capture devices have different abilities), but 720p60 is more common than 720p30, so it may simply may not be, and likely is not a supported format. 

You also shouldn't have to speed it up (as it should play back at it's base frame rate). This is common on higher end camera systems such as RED and ARRI, where you can set your base frame rate of say 24fps, but record 240fps thus it would play back at 1/10th speed. 

The base frame rate is encoded into the file's wrapper, and most if not all programs would recognize that for correct playback.