They may want to make absolutly sure no wifi signal emit from the device. Turning it off in the setup page is definitely not enough.
The wifi chip may emit signal during boot. The device may get accidentally reset in the field. SpaceX may push an update that messes with the settings.
I mean, more power to them, certainly, but WiFi emissions seem like the least of your concerns when you're operating an antenna for satellite comms. There will be no shortage of side lobes at Ku band for anyone who cares to listen.
The wifi chip may emit signal during boot. The device may get accidentally reset in the field. SpaceX may push an update that messes with the settings.