The difference in data use from screen size is in the frequency and duration of use. The Internet is just a better experience on a notebook with a bigger screen, mouse, and keyboard than it is on your phone. That leads to more time spent on the Internet and more data use.
And that is precisely why they EXPLICITLY say that feature is not part of the plan and offer a that service separately at a charge.
For everyone saying you paid for this, no you didn't. Most home ISP's start at @ $30 (and actually a fairly similar speed). Mobile costs more, and is worth more to the consumer. If we had "already paid for this data" to tether, the plans would be $60 or more, not $30. It's blatantly obvious the plans were not sold as an ISP replacement and were not priced as such. Therefore, no we have not already paid for that data. Or maybe it makes more sense just to say you did not pay for mobile service for your laptop.
Or if you want to be technical, you did not pay for the service to connect a PC to VZW network. The service for which we paid and which VZW provides is connecting our phone. They do not limit the data to that service. We're not paying for data - we are not metered - we pay for mobile data service.