Right, that's governed by your contract with Coinbase, and is the service they offer.
> Now every btc owner gets bcc.
No, everyone controlling a BTC wallet is now also controlling a BCC (or BCH, I've seen both used as the abbreviation) wallet with an equal number of coins.
I'm guessing rights to coins on alternative chains that Coinbase ends up controlling due to BTC forks are not covered in your contract with Coinbase.
There is a difference between ownership and possion. Technically (in the meaning "how it is realized in computer code") bcc is given to the person possessing btc. But by the law the owner is entitled to the profits of a thing. But coinbase does not own my coins, it merely posses them.
Yes, ownership is a matter of law. By what law does the act of a fork which grants possession of BCH to controllers of BTC wallets grant ownership to anyone else?
> But by the law the owner is entitled to the profits of a thing
That's, at best, imprecise. I own money, which I hold on a bank, which contracts a certain rate of interest (which may be zero) to be paid on it while I hold it there. I absolutely do not own any profits the bank is able to realize through holding the money (any more than, beyond those contracted, I am liable for any costs associated with their holding of money); indeed, their ownership of the net profits realizable by having the money in their hands is central to their business model. (There are restrictions on their ability to use the money they are holding to prevent undue risk of them not being able to pay my money back, but if they gain a windfall, I don't share in it.)
The real question seems to be legally whether BCH is a gift to those owning BTC or those possessing it. In the absence of very strong evidence to the contrary, I think the law is likely to say that who possession was given to also determines who ownership was given to. If I give a thousand dollars cash to everyone I see wearing a green hat, and one of those people had borrowed the hat, the owner of the hat is going to have a difficult time arguing that they own the thousand dollars.
Bch is not a gift. Who should have gifted it? Who is the previous owner? How did the previous owner aquire it? It is a "fructus" of btc. And that belongs to the owner.
This right can be signed away, with life estate or usufructus. But that's a big deal. That can't be somewho implied in a sentence hidden somewhere in a long ToS.