There are phone thieves in San Francisco/Oakland literally driving Audis and BMWs. Trucks bring in a lot of value. A tow truck is also inconspicuous looking, people can think its a repo truck or towing someone parked illegally.
A (new enough) tow truck screams "I have an ongoing business relationship with people who have badges, guns and letters of marque that give them permission to ruin and/or end your life at their discretion" which is really why none of the people who would perhaps bother a common thief or stick their nose in anything out of the ordinary will get near a tow truck.
You could roll up to a parking lot with active security and snatch something with a tow truck and they won't bother you most of the time whereas if you rolled up to engage in swapping a battery or some other legitimate repair they'd probably at least come over and ask what you're up to.
If you want to prevent theft, you have to make stealing "expensive" enough for people not to bother with it.