why can't the ama sanction that a ama field is a ama field and visitors to a local area have the right for a fee to use any ama sactioned flying field ?
Because simply put, the AMA isn't paying a thing for the field, so they have no right to mandate in any way, shape, or form who uses it or how much they pay to use it. That's up to the local land owner. The AMA extablishes standards for safety and structure of the club. I don't see where forcing clubs to let visitors fly being mandated by the AMA falls into this category.
I travel a LOT. Everywhere I go, I find the local flying club. EVERY club I've visited, after talking with the flyers for a while, has told me "Next time you come, bring a plane". I've even had club members offer to let me fly their planes. And nobody has ever asked for a contribution to the club.
If a club sees a benefit in a daily use fee, they'll adopt it. Personally as a club president I don't see it as being something we could reasonably establish. Doing so would now make the club liable for sales and use taxes, addtional paperwork. If a site is locked, how would a single day user get access? Or are they supposed to provide a combination and have to change the lock after every day user?
If you want to fly at a club, pay the membership. If the club won't let you join, why would you want to force yourself on those that don't want you?