I don't ride an ebike because of my disability, but because of my disability an ebike does allow me to go further and tackle hills I might otherwise avoid. I'm pretty sure I don't ride anywhere near as far as any of the non assisted bike riders around here, but it does level the playing field a bit.
So, from my perspective, if I were to use a bike park, and my means of using that bike park was an ebike, and they wanted to charge me more (than a non-disabled person) for the privilege, then yes, I would feel this was unacceptable and potentially direct discrimination as I'd probably be cycling a shorter distance, and slower than most.
Gaz