In this system you want to be under the cap because you are not forced to buy a contract. I understand just it doesn't apply to hosting. I wouldn't have interpretable power contracts for a data-center. The power rate does go down even more if your under the power contract but the discount is not attractive enough to compel me to operate under those terms. If you're are running a super heavy power load business that can handle small cut-backs, I can see that being a good thing.
Companies choose to operate on the interruptable schedule in exchange for lower rates. There is always a contract involved. Your choice is interruptable schedule for a lower rate OR non-interruptable schedule for a higher rate.
Why would you pay more for power on Bitcoin mining then you have to?
Of course this is mostly academic it would take a very long Bitcoin operation to even be given the choice.
I understand just it doesn't apply to hosting.
A datacenters will CHOOSE to pay MORE for power in return not having an interrupt provision.
A aluminum smelter (or Bitcoin farm) will CHOOSE to pay LESS for power in return for having an interrupt provision.