I'll tell you why this is not an 'exploit'.
Firstly, SE set the vendor price for Dawn Drops. No one figured out any system work around to make them sell for more. Since the addition of grand companies, the dawn drops have always been there under recruit. This was a very long time ago adition, it is nothing new. Though at the time, there was no way besides the few quests offered to get seals, so whos going to waste them on dawn drops? Later on down the line, new ranks are added and ways to get seals constantly are added. People have been playing this for a long time and clearly it didnt take people very long to pick the dawn drops as the item to deplete seals on for this event. So I would say people have done this in the past when there was nothing else they wanted from the GC and were maxed on seals.
Now SE adds the Atomos NM event, where you can get over-aspected crystals/clusters and exchange them for Seals of your Grand Company. At first, they messed it up to where you could get more than one cluster. That was their blunder and they fixed that already. However, SE set the exchange rate for these items themselves. 1 cluster = 3000 seals, 4 crystals = 1000 seals. SE also set the drop rate and amount of crystals dropped from atomos, at first being 3 to 5, now being 5 or 7. They also set the voidstone to be left up for a certain amount of time and to be multi-examinable. No special tricks by players were needed to increase the amount of crystals you get, amount of crystals you have to turn in, or amount of seals you get from turning in crystals.
The NPC that gives you seals for crystals wont let you turn in anymore if it will put you over cap. So any player that has everything they want from GC and wants to get that 500k seal ach, has to do something about it. So of course they would Take advantage of the Dawn drops that have been there since the addition of the Recruit rank, and have always been purchaceable with seals, and vendorable as a means to convert seals to gil.
Its not an exploit, its something to take advantage of while it lasts.