I believe the default state, in the absence of any law whatsoever, is that you can do whatever you want. Nothing is forbidden by default. You could assault another person, and it would be legal absent a law forbidding it.

Then there are laws that prohibit things like broadcasting radio signals without a license.

Then within those very same laws there are exceptions allowing certain transmissions WITHOUT a license, in specific circumstances.

The "default" is that things are legal unless they are made illegal, and I think it's pointless wordplay to try to argue the default state is somehow the law prohibiting unlicensed operation, and not the part of those same laws that allow unlicensed operation in particular bands with particular transmission power limits.

I understand the point you're trying to make that some parts of U.S. law say that all not-explicitly-mentioned radio transmissions are unlawful, but that's not meaningfully different from drug policies that say that all "harmful substances" are unlawful and so forth. They all just boil down to what the law defines as being within its own scope.