The intent is that you have to pay to use the WiFi. Even if you find a clever technical way to circumvent that that, the judge will see what you were trying to do: avoid paying for the service is offered. The court is not a computer and a judge will use their human brain to make a judgement of your intent.
Is it also theft if I run sshd on udp 53 and I happen to be able to connect?
How about if I run sshd on tcp 22 and it's not blocked?
Is it illegal if I just want to see if a dns change I made has propagated and I query an A record?
It seems obvious (to me) that a judge would say "It's not theft if you're giving it away. If you have a problem with how people are using your free service, add restrictions. Case dismissed."
I would hope that the court uses their human brain to make a judgment of my intent and the intent of the service provider. My intent is to have free DNS access to communicate with my server. The service provider intended to provide a public access point with free DNS and no restrictions on its use. The conclusion should be obvious.
