There's a 5 gb/month cap in your contract. They have the right to charge you for data used beyond that, or to terminate your contract (or both!). You agreed to that when you signed your contract.

Even the aircards and paid tethering plans have this limit. It's the same on VZW. I laugh when I see people using VZW aircards as their primary home broadband because I know it's only a matter of time before big red bends them over.