Adding a liability is the way you defer income on revenue. We have no idea if the the amount of revenue received from the sale of tickets not yet used is material. An entry might not be required if the amount isn't material.But the difference is, a magazine company still has to deliver a physical product to the pre-paid customers that costs them real money to produce. What is the cost differential to Disney to allow a person entry into the parks? It's a much more vague concept with a liability that is very, very small, as they don't have to pay another CM just because that one person entered the park.
If they want to defer the revenue, maybe that's an issue, but I'm a long way from my accounting courses in college...
I'll speculate the reasons given by previous posters (for the rumored change) would be more significant then just accounting issues.