Curious. Is that very recent? I was in Amsterdam last year and while I remember the ticket validators, I don't remember them being mandatory; I think we've even passed them to cross the station to the other side, to catch the ferry.
In Utrecht they recently closed the main corridor from one end of the station to the other (which, unlike the Amsterdam station, also splits the city in half).
Fortunately, they left a relatively narrow outside corridor open. Still feels odd to have to check in and out again when you're coming from or to some of the main busses, or take a detour.