I can see why this isn't allowed (seals / circles in free box's, promotional rewards, etc etc). It can be a risk when trading charms for these items off players, which is why it's best to keep an eye on who you trade with & ensure you're trading with someone trustworthy, or just buy from the market. As the saying goes, if its too cheap, its probably too good to be true.
I don't see why it isn't allowed. Yes you get seals n circles free from boxes and people sell them for charms anyway. It would be much more convenient for premium clans if the seals/circles/chimes were tradeable. Most times theres not a spot for the seller to get into clan to drop and if they miss drop then you have sit and wait. Charms are free and tradeable as with everything else so why not the premium eb items? When buying a seal with charms I don't think anyone cares if it was purchased from market or got from event and boxes.