we are not arguing about RECEIVING, it's TRANSMITTING over someone else's network that is the problem.
It's a problem both ways. Unauthorized interception of communications is illegal. You can't effectively steal bandwidth unless a transmission is involved as the communications protocols demand activity in each direction. Most devices with an integrated Wi-Fi device have a means of disabling it. That is a requirement to operate the device on an aircraft. Enabling the device without prior authorization to connect to a network is an intentional act.