So they should deliver 1 Mbps service over a 100 Mbps last-mile connection, so nothing is throttled, and there's no congestion, and there's no incremental cost to your bits.
Instead, every user, and the companies providing the service, prefer an over-subscribed service that cuts user cost. The only question is where do they put the line. Too much OS and the performance is bad. Too little OS, and the cost is too high. One of the solutions is more OS to cut costs, and bandwidth caps/costs to deter usage from the "worst" users to benefit all. The high-bandwidth users hate that solution.
Other people may suffer at the hands of your use of the total bandwidth at your area of the Internet but the costs do not change
If the suffering users leave the ISP, then it most certainly does change the Internet costs for the ISP (or at least the # of customers to spread the cost across). There is a real scarcity. There's not unlimited bandwidth everywhere, and it has costs to provide.