Apparently the reason is "[tobacco growers] contribute a lot to this country" [1].
You know, as opposed to all of the non-federally-subsidized occupations that contribute literally-anything-other-than-lung-cancer to the country...
I'm assuming the $1.1 million Tobacco spent keeping Richard Burr in office helps him not think too hard about such things [2].
[1] http://thehill.com/blogs/floor-action/senate/301645-senate-r...
[2] http://www.mcclatchydc.com/news/politics-government/article1...