I'd imagine Wi-Fi Alliance might have some biases here.
Since in-flight wifi is just a satellite connection, I'm sure the overall available bandwidth is fairly limited. Charging for wifi is a good middle ground for airlines as a way to make a few extra dollars but also to keep usage to a level that the uplink can handle.
Are their any airlines that currently offer free wifi? I know that SWA did during their testing phase a while back (which only consisted of 3-4 equipped planes) but they eventually started charging for it like everyone else.