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.
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.
You're probably right that the physics of the magnet don't make it as useful. That makes me wonder how thoroughly Apple has investigated it.