I'm fairly confident that this is not a "grey area". When something is specifically prohibited in a legally binding document that you agreed to / signed it is not "grey" it is, quite literally, "black and white". And, as the other posters have pointed out it is almost always specifically prohibited.

I don't know about you, but I've never signed a contract for residential Internet access. I just call them up and give them a credit card number and a few days later have a box with a live Ethernet port show up in my apartment.

I'm sure they could cut me off if they didn't like what I was doing, but they could cut me off at-will so it's not worth trying to read their mind.

In this case, it might be useful to indicate how fast your connection speed and network is, to give people more information if they decide to buy access.

What about having bitcoin payment for x minutes of access to the hotspot? :)