In case you haven't noticed there is no sales tax on groceries. I can't speak for everywhere, but this is the case in Florida and most other states.
This however does not hold true if the food was prepared. For instance, the next time that you buy a sub made in Publix it will be taxed. If you buy the ingredients separately and make it at home, it will not be.
This is another prime example of why it's much much cheaper and healthier to eat at home rather than to eat out.
Sales tax (7%)
Marked up food (100-300%)