Again, if an insurance company wants to do business in Missouri now then all they need to do is register with the Missouri insurance people and meet their requirements. But if they choose not to do business in Missouri but instead I go to them and ask them to sell me a policy will they be forced to do so?
No. That would be in conflict with getting government out of the health care system (at least where private insurance is concerned).
Also, while you are correct that Blue Cross, Aetna, etc can have presences in many of the states, I don't believe businesses are able to pool across state lines. This is one of the suggestions made by the NFIB as noted here:
Another suggestion by the NFIB is to allow small businesses and inviduals to 'pool risks and purchase insurance across state lines.' Presently, large businesses, labor unions, and governments generally 'self-insure.' This allows them to pool their risks across state lines, thus developing 'larger, more stable risk pools, thereby lowering costs and reducing uncertainty.'
http://www.thenewamerican.com/usnews/health-care/item/22460-nfib-offers-alternatives-to-obamacare