The fact that it appears to be happening primarily to jailbreakers is a coincidence. Most jailbreakers don't upgrade so they can avoid issues like a tethered JB or loosing their unlock...etc. Those that don't jailbreak aren't worried so they take the latest updates as they come. So a large number of those getting this message just so happen to be jailbroken. Strictly coincidence....nothing more.
The last time AT&T texted with a software update, there was a major bug in the baseband and they were attempting to get all users to update to resolve this issue since it was locking up channels on the tower IIRC. So this and the Engadget article explains why they are sending the text...not some evil plan to get all the jailbreakers on stock firmware.
Of course if people refuse to update, whatever this issue is will continue to cause problems. I can't speak to this update, but I know in the past when AT&T did this, there really was a bug....it wasn't some scheme.
