Legal liability. Those who fear being blamed for misuse of their public Wi-Fi signals are said to be protected under a "safe harbor" doctrine akin to that protecting Internet service providers. In other words, they're likely not liable for the mischief of porn purveyors or music pirates.
Sure, you could win in court, but that won't stop you being sent invoices by lawyers looking to make a quick buck, accusing you of downloading porn. I doubt the police will bother to check if you have this service before arresting you on suspicion of downloading child pornography either. You may eventually be found innocent but by that point the damage has already been done.