I have seen the same problem, where the front seats are the ones that are open for late comers. But, this is because most people really only grudgingly go to boasting sessions and do not want to sit directly in front of where the hounders are going to see them, not because of any arrangements. I always liked to sit somewhere near the middle myself, and off to one side, so I would not be directly in their line of sight.
What I have seen is to the effect of 20% of the witlesses are early (being defined as being in the building with their seats reserved discretely before any mention of the boasting session is made). Another 60% come just in time (as defined as coming into the building sometime during the introduction, or still looking for their seat when the introduction comes). The other 20% are blatantly late (they are not in the building until after the first of the talks is in progress). Poor organization is cited as the reason--but, if you have children that do not want to attend the boasting session, and you get off work at 5:30 or 6 PM, I do not see how they can consistently be on time.
As with being there at all, I believe more people would make the sincere effort of being there on time if the value of doing so exceeded the cost. With people having to bring children in, after not being at home from work until well after 5, the cost can get pretty high. If the value does not exceed that cost, people are not going to find it worth the effort--and often they are late (or they don't go at all).
Yes, if they are going to start harassing people for being late, I recommend not going at all.