You mean "expensive" and "cheaper" wasn't clear enough? Really?

"No capacity controls" means you can book a seat even if it's the last available seat on the plane, when AA would be looking to sell it for big $$$.

"Capacity-controlled" means that AA has determined that it likely won't sell all the seats and is willing to grant an award seat for fewer miles (in this case, just 12.5k miles).

If you want to spend 30k miles, you're free to do so. But since the "cheaper" option is available for 12.5k miles, I don't know why you'd worry about the more expensive option.