Isn't that just a dividend at that point? In the real world investment funds have fairly complex dividend rules (how much is reinvested in the fund, how much is skimmed for fund manager fees, etc), and regulations have managed to force such funds to disclose all of those policies up front (prospectus) and keep them up-to-date.
In the other fork in this discussion thread, the banking analogy: we're talking about an exotic interest payment of sorts. Again here, banking regulations require banks to disclose interest rates and payment details and management fees up front for accounts. But banks in the real world are largely allowed to reap the majority of profits from an investment that goes well, and doesn't have to apply much if any of that back to the savings accounts from which it utilized stored funds to fund that investment.
Which is to say that Coinbase may be doing nothing wrong according to either real world analogy, with the big difference being the "wild frontier" of existing in an under-regulated space with fewer up-front disclosure guarantees.