The precedent was set in an earlier case, and the precedent is that "data transfer can be considered property." That is, if Google is found to be 'stealing' data (ie, using cell phone resources that the user gets charged for), then they can be held liable for that.
The jury trial in this case found that Google was "stealing" through transferring (ie, giving the user a higher cell phone bill), but that was a question of fact determined by a jury, so it doesn't set a precedent on questions of law. There's nothing magical under the law about transferring data while the phone is idle, but the lawyers managed to convince the jury that it causes harm, and Google's lawyers failed.
This article has a lot more detail:
https://www.reuters.com/legal/...