Here's Wyoming's law:
"Each application shall be accompanied by a surety bond, irrevocable letter of credit or other similar security device acceptable to the commissioner in the amount of ten thousand dollars ($10,000.00) or two and one-half (2) times the outstanding payment instruments, whichever is greater. The commissioner may increase the required amount of the bond or security device to a maximum of five hundred thousand dollars ($500,000.00) upon the basis of the impaired financial condition of a licensee as evidenced by a reduction in net worth, financial losses or other relevant criteria."[1]
For most money transmitters, this isn't a big deal. They don't hold the money for long. The whole point of the business is to get funds from A to B quickly. So they don't have a large obligation to their customers at any one time.
Coinbase's "hosted wallet" service makes them a depository institution - customers have funds on deposit with them. Given the track record of Bitcoin "hosted wallet" companies, most of which have failed, requiring reserves as a consumer protection measure makes good sense. Coinbase could post a bond from an insurance/bonding company, but they'd have to convince the insurance company of their financial soundness. That they can't easily do so is a bad sign.
Coinbase should still be able to do their business of converting Bitcoins to dollars for merchants. They only hold the money for a few hours, and should be able to have reserves for that float.
If Coinbase became a bank or a broker, they'd be subject to US Federal regulation and audits, but would no longer need state licenses. They're trying to sleaze by as a "money transmitter", while acting as a depository institution, and it's not working out for them.
[1] http://legisweb.state.wy.us/statutes/statutes.aspx?file=titl...