"This shop seller is either naively and perhaps even innocently ignorant, or just doesn't care - as others have pointed out, thats entirely their prerogative - but its also understandable that in such situations others may become insulted."
Exactly as the text of the post says - she is within her right to sell whatever shirts she wants, whether they feature Hitler or Osama Bin Laden, so she doesn't care about other people's feelings - just about the money she makes from selling them.
As another poster pointed out, I wonder how she would feel if she had lost a loved one in for instance, a car accident, and somebody printed a tee shirt of the disaster scene accompanied by some crass comment, and put it on sale to the public?