That does seem odd. But perhaps only one way from each park. For example, if you are spending your day at IOA you can only take the Train back and forth but whenever off the train at the Studios side you cannot enter the turnstyle into the park without a valid ticket scanned. You can only get back in line. Same being said from Studios to IOA. You can ride Hogwarts Express to IOA but when you get off on the IOA side you can't enter the turnstyles there to the park without a valid ticket, you can only have a re-enter line.
Overly complicated in a way. But that is a way it could work. I wish they would of just kept it so you can only go if you have a valid two park ticket. This way it would be an incentive for people to pay for the other park as well as keep the lines down for Hogwarts Express.