So, I get what you're saying and don't disagree. I've been to restaurants requiring a suit and tie, and clubs requiring nice clothes and no baggy pants or baggy shorts, etc.
The difference between those places and what you're talking about? At each of these establishments, the attire was checked at the door.
If you came into a suit/tie place with shorts and a T-shirt, you wouldn't have been allowed to walk through the door.
The club I went to with a dress code? The security pulled me to the side before I could ever enter and questioned the shorts I had on and whether they were 'baggy'. They weren't baggy - they were just long and I had to show them that the shorts were up around my waist.
So, Six Flags might want to use the excuse of "dress code," but if they let her in wearing what she was wearing, then that is on Six Flags for not enforcing the dress code at the gate.
Sure, people can change clothes once in, but that's not what they claim happened.
So if they don't want short shorts being worn, they need to check these things at the gate. It wouldn't be difficult to do either - I've waited in line at the gate to Six Flags and on a busy day, it can take forever. Plenty of time to correct these issues before.