There is no priority between people clicking "need". They both have the same priority, it's not ninjaing.
Everyone who queues up for LFD implicitly agrees to the loot system whereby people choose for themselves what priority to opt into. There are no permissions given by other players or even a consensus. People act as individuals unless they discuss some agreement during the run. And even if there is a discussion, players are not bound to any sort of consensus if they don't agree with it. If you can't handle people not adhering to the same loot priorities as you, then don't use a system where you have no control over the loot. Make your own group and set up your own rules with people who agree with your loot priority.