Not sure if anyone has mentioned this yet, but it is entirely possible that the admins decided to make it an official faction only event in order to avoid what would be an immediate influx of the more PVP oriented members of the community making Indie ships to go at each other. We have all witnessed what happens in the past when events are made with allowances for indie players to join the frey, and it always turns into one side complaining about unfair numbers and the inability to do anything in the event given the skill, numbers and blah blah blah, #PVPwhoreissues.
This is of course how I see the admins decision and personally think it's a good one. If all of you that are complaining about not being able to go pew pew as an indie in this event were to actually join the official factions involved, then I'm sure that would be better in the long run, both for the faction, and the players. Of course, you won't be able to join both factions and shoot at whoever is online whenever you please, which is yet another issue that arises from indie allowed events. Someone spots a player on from one side of the fight and they log an indie vessel on the opposing side, then, once they get their blue, they spot someone on their current side logging, and thus, log for the other side and get yet another blue. This is an issue that has been around forever when it comes to events and pretty much any real situation with a lot of ships on either side, thus, I'm sure the admins made this event in such a way as to avoid such complications and anger.