In that case I also see no difference in owning shares in MPEX which would mean buying even 1 share makes you liable for all its debts public and private and satoshidice since both are "unregistered companies"
The fact they are misleading people into unlimited liability is unfortunate.
I honestly don't know how they're set up, so I hope that MPOE-PR will come along and clarify that. There are ways you could set it up so that only some shareholders had liability without setting up a company (by issuing different types of shares, for instance, so that investors were basically limited partners) but as a general rule you need to take an action of some kind to limit liability.
It bothers me that so few enterprises do go to the trouble of incorporating because it's not particularly expensive and a lot of Bitcoin enterprises find themselves controlling large amounts of other people's funds within months of opening. When enterprises are skimping on standard business start-up costs, it always makes me wonder how else they're cutting corners.