Moody if you consider event items different to dropped seals how do you feel about event chests which are dropped from EBs? What if someone opens a chest they got as a drop from within that clan and receives a seal. It's a rare drop but it was attained from the clans effort with similar logic to seal drops.
I wouldn't consider receiving a seal from a chest the same as receiving one from an EB. Aside from the seal/horn you can't share a reward you gain from opening a chest with your clan. So in that case the seal/horn should be yours to keep and use how you want. Also there isn't a public record of you earn that from a chest, so I can't see any owner going out of their way to find out if someone's chest dropped a seal. Or at least I hope no one would do that lol.