"We don't have the factory capacities at the moment, therefore your car is missing the back seat" is no better an excuse than "It's cheaper for us to leave out

"We don't have the factory capacities at the moment, therefore your car is missing the back seat" is no better an excuse than "It's cheaper for us to leave out some parts, therefore your car is missing the back seat". There is no such thing as "surplus bandwidth". How much bandwidth there is is as much the decision of the ISP as what gets prioritized or throttled. If you make it illegal to throttle below available bandwidth, they will simply reduce available bandwidth (or even simpler, fail to increase available bandwidth). If there is more than one way to achieve the exact same result at exactly the same cost, it's completely pointless to make one of the ways illegal.