Earlier quoted context omitted.
American schools not only "don't try very hard to engage in best practice," that sort of thing is often explicitly forbidden. My cousins are teachers in Americorps and preparing their own lesson plans or using outside materials are not allowed under their employment contracts.
How old are your cousins? Americorps seems to recruit (only recruits?) twenty-something, recent college grads. The people I know who've gone to work for them were certainly in that age group. Other teachers I know (15+ years of teaching) seem to have a fair amount of freedom in their lesson plans and materials so I wonder if this is an issue with Americorps, younger teachers, or perhaps I've just known some fortunate…
After edit: The Americorps website is a mess, so I'm still looking around for information about where Americorps members might work as teachers.