If you're on a subscription plan, you're paying for a certain amount of maximum token consumption. Mass market consumers generally prefer this model to one where they're billed for actual usage. But making it work requires statistical estimates of how much people will consume, which often requires excluding third party tools that circumvent those estimates.

To use your analogy, if Shell sold you a subscription to fill up your Hummer up to 30 times a month, they wouldn't let you use that subscription to fill gas cans with a GMC logo taped to the side. They couldn't, without overcharging the people who just want to average out their cost of driving.