It's a free hotel room voucher. And the extra bonus is that due to the weather in Detroit, they have no idea if/whether any cabs will be avilable to go elsewhere.
It seems very strange to me, this happens a lot to me (the dangers of travel to Northern Michigan) and they often say "sorry that hotel is full". So they obviously have some way to monitor room availability - how can they give a voucher to a Diamond (or anyone for that matter), have you spend an hour getting to that hotel and then find out they won't take the voucher??
Keep in mind, they HAVE rooms. They are letting 'cash' customers check in. Just not those with vouchers.