It's hard to put the blockchain in a real world analogy. For my example I can imagine that the FED had the ability to issue money electronically and attach or detach currency based on holdings, which it could track.

Building upon the previous, I think a more proper analogy would be a bank claiming a currency issue for its own benefit by utilizing deposits in it's secure box service.

Now, the fact that there was no clause in the agreement to prevent this, doesn't mean that it doesn't violate ownership laws. If the intended recipient is the owner of the cash, he should have the power to decide what to do with the new cash. The bank wasn't given permission (ignoring regional law enforcement regulations) to access secure box contents on demand.
AND, the new currency issue, attaches to the cash (coins), and not the box (btc address). The fact that the cash was in the bank's box, only gives it the ability to access the cash and its attachments, but not permission to.

Coming back to BTC and forks, I would argue that since the original owner of the coins agrees with the treasurer that coins are just to be held secure, not spent, then airdrops should fall under the same umbrella. Since they were dropped to the treasurer only because of someone else's property, then the treasurer should abide by the rules treating them as BTC. Treasurer would not be getting an airdrops on the treasury address if it weren't for the treasurer coins anyway.