I just wanted to agree with BroganMc that the CM was wrong. You should not have needed to move (and would have been completely within your right to stay). I've had a couple close calls getting stuck from parking my powerchair on the grass to watch an event (not at WDW). There's no way I would have moved onto the grass.
I don't know the specific area, but from all descriptions, there wasn't an issue where you were parked. It's too bad you didn't ask her at the time or get her name. Any time you get singled out like that (especially separate from your entire party), it's worth asking why and if there's another alternative. Hopefully she's since learned her mistake, but it's unfortunate that you lost out as a result.