They could just say "4gb" or "250mb" and reserve the words "unlimited" until they truly are. I'm on Three and use 20-25gb/mo. They say it's unlimited, but it certainly isn't limiting.
It can never be "unlimited" because there is always a limit to how much data you can transfer in a month, given a theoretical maximum bandwidth of x Mbps for 4G networks (actual achieveable transfer speed even less). The correct term is "unmetered".