"by identifying abnormally heavy users and removing them" - That costs money, ergo, theft. It's like if someone had to hire a security guard for a vending machine.

Or even just let those users alone. Users aren't stealing service if it's not even the same service. It's much slower than buying wifi from the captive portal.

DNS tunnelling is not fast or convenient. Places deploying captive portals have probably looked at the risk to their business from it and have decided not to worry about it.

I can't believe that using a slow DNS connection, intentionally made public, to tunnel traffic would be considered theft or criminal.

How many free samples do I have to eat before I'm a theif? I don't believe I'm a thief until the offer for free samples is rescinded.

I would imagine at the very least you would degrade DNS resolution times for legitimate users since there would be a lot more requests than usual

That’s exactly like bypassing the electrical grid.

It’s like having a “free” street light and, instead of just enjoying the light, you pull its cables and plug your AC in.

The free service is just for the light.

Can I use my solar powered calculator under a street light? Or is that theft of service too?

The light is free. The electricity isn't.

I suppose adblock is theft too?

Opinion differs, but many ad-supported sites would say yes. I'm not sure if it has every been tested in court. "Fare dodging" might be a better concept to compare this to.