'I have so many state standards I have to teach concept-wise, it takes time away from what I find most valuable, which is to have them inquire about the world,' said one teacher."

They're doing it wrong. It should be "How do I teach the students to inquire about the world and still meet the state standards?". When you have a well designed curriculum, that's what happens. A side effect is that the you don't need endless practice tests to pass the state tests, at worst, you just loose a day when the students take those tests.

This will never happen as school administrators are extremely risk adverse. They will never be able to accept the risk that the reason their students didn't do well is that the open source textbook they used didn't meet the state/federal curriculum standards. The state/federal education agencies will also never certify that any text book meets their curriculum standards.