None of those reference frames are accelerating.

The difference is whether there is a changing velocity or not.

I'm going to assume that you're defining acceleration in that second statement, because I'm not sure if you are and "changing velocity" is literally what acceleration means. In any case, both acceleration and velocity are vectors, both have a direction, and so a person's velocity sure as hell can't be constant when they're going in circles. Ergo, acceleration. I mean that's what force is, mass times acceleration, so if you move and you can feel it you're accelerating. Earth has gravity that can more than cancel it out, but we can't say the same for rides.

Somebody smarter and with more energy than me can probably come up with a rough estimate of the g's being pulled in each picture (ignoring gravity).

Edit: looks like someone did!