Although data caps on internet usage aren't popular, most ISP’s (internet service providers) use them.
A terabyte to four terabytes is a generous data allowance but a very small percentage of subscribers will exceed that usage. Subscribers who use more bandwidth cost more to have service provided to them, therefore they end up being charged more.
Data caps also help support tiered subscription plans. This means that an individual that only uses the internet to check emails can choose a plan to pay less than an individual who uses large amounts of bandwidth for streaming and gaming.