From Ronald Bailey's article in Reason:
As analyst Marlo Lewis from the free-market think tank the Competitive Enterprise Institute points out  such a ban on exports would violate World Trade Organization rules requiring that imported and locally-produced goods to be treated the same once foreign goods have entered the market. In this case, one way to get around this rule would be to also ban the export of refined products made using domestically produced crude oil. Let’s hope that Rep. Markey doesn’t decide that this is a good idea. In addition, Deputy Assistant Secretary for Policy Analysis at the Department of Energy (DOE) Carmine Difiglio noted  [PDF] in a June 2011 memorandum that since crude oil imports from Venezuela and Mexico are declining Canadian crude would help make Gulf Coast refiners less dependent on imports from countries with less savory regimes.