http://www.nbcnews.com/business/u-s-reports-9-7-billion-loss...
http://www.nbcnews.com/business/u-s-reports-9-7-billion-loss...
The ability to produce a car and similar heavy-industry type production is a strategic asset (which is I think one reason why the French government has been so supportive of it's native car industry for so many years even when French cars didn't sell well outside of France).
It's a hard balance, support an unprofitable industry to protect a capacity you might one day need.
If I recall correctly the US has(had?) a policy where american flagged cargo ships that mostly traded in and around America had to be built in America, this made them much more expensive but kept the major ship yards in business as it was seen as a bad thing if that capacity was lost.
The government chose to sell its shares underwater for political reasons. Americans, unlike Europeans and Canadians, don't long tolerate partially nationalized companies, even if it makes them money.
The $10b loss is probably a lot smaller than the impact on the deficit if GM were allowed to go bankrupt. Keep in mind that most US deficits these past few years were not due to the stimulus, TARP, or any Obama legislation - they were due to the automatic stabilizers (medicaid, EI, food stamps, etc.) that have been law for decades, and skyrocketed with the financial crisis due to the vast numbers of people (over 1.5% of the entire US population) dropping out of the labour force between late 2008 and late 2009. It's up to 2.8% now.
Temporary spikes in supply (or demand) can drastically distort the value of any asset.
[EDIT: clarity]
I'm not convinced that the market for GM stock is sufficiently liquid to mitigate that.