Just some thoughts.
One of the first cars BMW built at the Spartanburg plant was the newly introduced E36 in 1992. Go research the quality problems of the '92 E36, especially the build quality of the interior. The quality of the early production Z3 interiors weren't much better.
If BMW has been so successful with the Spartanburg plant in the US, why not build another plant in the US? Maybe Mexico gave them a better deal and has a better business climate to make a long-term investment? Maybe the current administration, which claims to be so pro-American manufacturing (as long as they are "green" jobs), has made the future American business climate unattractive to BMW? Pro-union labor, high corporate and personal income taxes, poor healthcare plan, poor (disastrous?) energy and fiscal policies; maybe these things were part of BMW's decision. Don't blame BMW, blame your politicians...
Vote with your brain this November, not your guilt.