It is illegal, that's true. I just learned this myself (thankfully, not the hard way!)

I'm a serious recovering felon when it comes to this, during my crime spree days, I would buzz around a town and look for a signal to jump onto for a few minutes. I am now cured of my compulsion to commit crimes.

I still use coffee house and hotel signals because if they don't want them to be available to just anyone, I know they'll restrict access, and many already do so.