You're reading the data wrong. 7.3% of minimum wage workers have a bachelor's degree or higher.
I assume you were looking at the number 13.5%, which is the percentage of minimum wage hours worked by those with bachelor degrees alone.
Another way of looking at this: 30.9% of Americans over the age of 25 have a bachelor's degree. There are 122 million Americans 25 or older. That means about 38 million of them have a bachelor's degree or higher. There are 279,000 Americans with bachelor degrees or higher that make minimum wage. That means a maximum of .73% of Americans with bachelor degrees make minimum wage. Adding younger people with degrees dilutes the number further.
56.7% of Americans over 25 are somewhere between a high school degree and a bachelor's. If we do the same math above, we get that there are 69 million Americans with a high school diploma but not a bachelor's. And 2.5 million of them make minimum wage. That's 3.6%, which is a difference of about a factor of 5.
Dealing with people between 18-25 probably skews the data towards a lower percentage of minimum wage workers more in favor of those without a bachelor's, but I think the point still stands. Unfortunately the BLS doesn't subdivide the data by both age and educational attainment.
Population by age: http://www.census.gov/population/age/
Educational attainment: http://www.census.gov/hhes/socdemo/education/