There's no arbitrage in what sense? The article I read that called it arbitrage described it like this: "[Hedge funds] borrow Bitcoin, deposit the coins with

There's no arbitrage in what sense? The article I read that called it arbitrage described it like this: "[Hedge funds] borrow Bitcoin, deposit the coins with GBTC in exchange for shares that are more valuable than the coins they bought, and they pocket that profit by selling the marked-up shares after a six-month lockup period expires." Unlike an ETF though, the shares can't be destroyed. And suddenly the premium went away, so probably some of the people who were counting on it to remain have quit now.