In English, everything that is required is allowed, by definition. It's what you'd call a tautology -- a statement that's true by necessity or by virtue of its logical form. Like it or not, it's true, and the test authors believe it to be true as well.
Roël, note that while you were required to go through security to board the ferry, that is also the one place via which you are allowed to enter the ferry. You are therefore allowed to go through security to board the ferry. Since you're not allowed to get on the ferry any other way, if you want to go on the ferry you are required to go that way.