I don't do it, so I don't have to justify it.
I like the rationalization given above. Since they provide unlimited data they can steal it.

That is essentially, I can do what I want, and I don't care so FU.
It is a rationalization based on a sense of entitlement that is not deserved.
I guess if I pay $10 a month for lifeline cable and steal all the premium channels it is not stealing either, because I am paying for it.
With AT&T they offer a specific plan for tethering that costs a certain fee. They in no way shape or form make available as part of the normal data usage the ability to tether. That should be clarity enough for anyone to know what they are doing is wrong.
People will justify pretty much any crime. Serial Killers and Mass Murderers can justify their actions, so when people steal from AT&T it is not hard for them to justify it, even if they are wrong.