You're forgetting the cost involved in porting and continually maintaining the driver for each and every new kernel that pops up. The profit margin is reduced by the salary of the person porting the driver plus the cost of the benefits, health insurance, and the operating costs of the company.
A junior level programmer could easily cost the company at least $75,000 just in a yearly wage, assuming it is a decent company that provides _average_ healthcare in the United States you'll have to double the wage to find out how much the company is going to spend just to keep the entry level guy employed and healthy. Not to mention the cost of keeping the driver ported to each and every new kernel.
That $400,000 profit is roughly cut in half.
Remember, nVidia doesn't have a problem doing this because of the huge contracts they have in the graphics workstation market, for a tablet company that has never supported Unix before that doesn't have any experience in supporting this insane platform the profit margin outlined above isn't worth the risk because it assumes that people will buy their product when there is no guarantee of anyone purchasing their product.
Now if the US Congress would listen to ordinary people and the American Medical Assoc. instead of the lobbyists of the health-insurance industry, and pass a health care bill that didn't suk donkey butt that profit margin would rise, but there is too much money coming in that is preventing decent health care reform.