Why Should People on Public Assistance be Drug Tested?

It upsets me when people preach that all people receiving public assistance should be drug tested before they can receive help. They seem to be assuming that everyone who needs public assistance is a down-and-out druggie who refuses to work and just wants the money for more drugs.  My father left our family when I was approximately 6 years old and went to CA.  To my knowledge, he never provided my mother with a cent.  I know how that abandonment affected me, but that's another story.  My mother was left with three kids, aged 6, 5 and an infant.  She was young, had only a high school education, and lived in a community with very few job opportunities.  Her parents helped as they were able, but she had to accept public assistance in order to feed her children.  Should she have been drug tested and treated as a criminal because she wanted to put food in her children's mouths?