You could possibly argue to the courts and business regulating agencies that it amounts to an unfair "hidden cost" that the average consumer isn't given upfront knowledge about. You pay full price for a ticket and assume that the park is doing everything in their power to minimize lines and push as many people through as possible so that you have more time on the rides and less time waiting in line. Selling priority passes basically throttles the experience of those who choose not to buy them, as some people wait less, but capacity is unchanged. So in a sense, you're selling a damaged experienced, but advertising it as a complete one in bad faith.
Same issue could come up if a park was excessively price gouging inside a park. Notice that there are premiums that are assessed on food in parks, but parks always provide ice water if asked, and the price of food is usually more or less reasonable and in the same range as other parks for standard park food. I'd imagine that a lot of regulation goes into it.
But, obviously at the moment the regulators and courts for now have deemed that they don't really care about priority passes at the moment, as every park chain seems to have a paid version at the moment. And for every chain other than Disney, there's no disguise: you just pay extra for the right to skip the line, there's no game to it like with Disney.