More importantly, requiring a software upgrade to enable that functionality would make it obvious that this is indeed of questionable legality and limit the market. Many people buying these watches (or the children dolls affected by the last round of bans) are not aware that these are in fact hidden spying devices.
Technically its not a ban of specific merchandise, since they were already illegal. Its more like, say, a police raid against an arms dealer (=> thats not an arms ban).
I'm not suggesting a first party would do it, but say a platform akin to Pebble, Apple Watch or Android Wear were to enable an app like that to be developed. What part would be illegal then - the device itself? The app? The app store? Restricting this seems somewhere between asinine and impossible.