I think there’s a national law because time zones are set at the federal level. Then within such federal regulations, states can decide about daylight time. That’s my understanding anyway.
Then the next question would be, can states decide on their own to have their own time zones. I don’t know the answer to that.
I’m OK with time zones being established at a federal level, because they do extend across state lines (and quite a few states are in two different zones), but DST should be left to the states. What people in northern states prefer might be different from what is better in the south. The question of DST shouldn’t be part of any federal laws regarding time zones.
Theoretically, this should be done with some regard to Zulu time.