This idea is based on dumb criminals. There is a vast difference between what can be done and what will be done.
Again...Me...N00b...
My understanding of Jasager is that it relies on the trust of the device/computer. If you say, "Yes I'm your home network" to every device, then they will connect to you and report their MAC. Correct?
Also in the US most data plans are limited to a certain number of MB's or GB's without overage charges. Most people switch over to the WiFi when they get to work/home/school for two reasons. First, they can limit charges for mobile broadband. Second, they get a much faster connection over WiFi than the crowded 4g.
This is still assuming that only phones have built in WiFi and only phones are stolen.