Yes, Coinbase should be required to support disbursal for every one, with the proviso that it either passes a certain price or passes a number of customers who want to withdraw it.
It's like depositing a dollar to a bank, and then finding out that it had been signed by Andy Warhol in invisible ink and asking for the specific dollar back. Sorry, you shouldn't have deposited in a bank. You can get a dollar back, but not the same one you deposited.
What you're looking for is more like a safety deposit box for Bitcoins. That's a whole separate product, with a different fee structure.
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.
The only argument you can make is Coinbase not givint a "reasonable" amount of foresight to users (who would pressumble have the right of advanced time/knowledge to convert to USD, etc and then back to BTC (on another "exchange").
I have no idea is this is true or not, but this is the only criticism of Coinbase you can have here.