The one way mileage charge varies by market.

Based on my experience, Florida is one market, and as such there are no charges for in-state one ways.
Where as any rental out of DCA does accrue mileage charges regardless of your AWD. (well at least the ones I have access to)

I believe its based on demand, as well as ownership. If the car is owned by a small franchisee you will encounter more resistance with a one way rental.