I understand fully why. The simple fact is, they should not have the authority to do so. Any attempt to do so, should be met with resistance and outright hostility.
Do you realize that the curtailing of water heaters would only need be done maybe 12 days a year (summer peak or winter peak)? No one at the electric utility wants to coerce you into taking a cold shower. It is a matter of not wasting resources. Electricity is very hard to store. It should be done on a voluntary basis by providing users with savings incentives.