Being allowed to leave the plane or not has nothing to do with direct. It’s up to the local authorities to allow disembarking or not at the stop point and it can change sometimes depending on the mood of the day. You have many flights like that in BKK for instance and, even for the same flight, sometimes you disembark and sometimes not. But in BKK you disembark most of the time. When this stop point is in the US (e.g. CDG-PPT via LAX), you always disembark.