Common sense says that nothing can be advertised as unlimited, because nothing on Earth is unlimited.
No, it doesn't. "Unlimited" has a very well-defined meaning that is obvious for most people. "Unlimited" usage of a 6 Mbit connection means that you can use the full 6 Mbit 24 hours a day, 7 days a week. (This works out to about 2 TB/mo.)
Obviously, this is bad for the network, which is why offering an "unlimited" wireless plan is an incredibly stupid idea. But that is what T-mobile did. Blaming their customers for their own mistake and calling them "thieves" is pretty low.