Agree, it is a feature that would be great to have in FCP for iPad and a bit of surprise that detaching audio is not available.
You can send Apple feedback here:
Feedback - Final Cut Pro - Apple
The only workaround I've come up with so far is to duplicate the clip, mute the audio in the donor clip and then move the copy underneath whatever video I want to place the audio under - as the video from the copy will be masked by what is above it - kind of treating the copy as an audio clip even though it has video.
MtD