Before I begin my explanation, could the above user named "Dirty_Nschos" please stop talking? An inside is a inside, there won't always be a way to find out if it was paid for or not. You're so desperate to make everything Ethreal related.
In my honest and humble opinion, Snapcraft Factions allows a lot of things that other faction servers consider bannable and I think at this stage claiming "it's what makes the server unique" is a force of habit.
The reason insiding is bannable on most faction servers is because its considered completely unfair. For example, a faction spends their time and money grinding for an F top paypal award and hours before the map ends, they get insided by a player and lose their position on F top. I don't think anyone here would want that to happen to them so that's why servers make insiding bannable. However, snapcraft isn't like that. In the past, factions have been insided and still been able to rebuild because of how long the maps are and most factions are playing for enjoyment and not f top payout. (I can see how this could be changing now since there's f top payout)
Things to consider if making insiding bannable:
Can you rollback chunks? Assuming the map is 6 months long, a lot of insides can take place in that time period. Can you keep up with insides?
Will all factions receive a rollback/get their items back? Even factions with 2k f top value can be insided. Or will you make it so that only insiding factions on f top page 1 is bannable?
Can you ensure the insider wont transfer value to an alt and log on a VPN later?
How can you, as staff, be sure it was an inside? We all know certain players will try to fake an inside to double their value. (Maybe some sort of chunk logs or something)
There would be a decrease in insiding but I doubt the number of people using legitimate methods to raid would increase.
I would also urge you to consider tackling the root of the problem. The reason people are able to easily get their alts into factions is because factions are always looking for power. How about, along with making insiding bannable, you allow players to have more than one alt. This way, they won't need to invite players they don't recognise for power because sufficient power is available from alts which will turn decrease the amount of insides happening.