Simple: it carries data. If the power cord on your laptop is yanked off for a minute it's usually not a big deal. If you disconnect the iOS device cable you could interrupt a data transfer. There is a higher risk to disconnection.
It is USB, and the host could lose power at any time (it doesn't have to be a laptop). Communication interruption is a likely case they should plan for.
A more probable explanation is that even with a magnet the devices are light enough that a yank on the cable would send the device flying before the magnet connection came loose.
