They do this in New Orleans too, and a couple other places. It has been in effect for a few years at ports with smaller terminals. The terminals are not large enough to have a lot of people checking in and sitting or standing around, so they ask that people arrive at certain times to minimize crowding, hoping that at least some will comply. It worked well in New Orleans when we cruised out of there last month. If you show up prior to your 'scheduled' time they won't deny you boarding.