50/50 Custody - Child Support Cases

Somebody please tell me why employment is not required by both parties in a 50/50 child support case? For example, Tom and Betty have a one night stand and Betty gets preganant. Betty takes Tom for child support. Tom is a hard working, middle class citizen, making an average salary and holds a steady job. Betty works at fast food restaurants off and on and has never had a steady job. Betty and Tom have a court ordered custody hearing and each party is awarded joint legal and physical custody, they each have their son the same amount of time.
Betty takes Tom for child support and is awared 15% if his GROSS income, not his NET income which is what he actually takes home. Betty is currently un-employed and she is held to income of $0 in the support calculation.
Now tell me why Betty is not forced to hold some type of employment instead sitting back, collecting support, and living off of the government.
Betty SHOULD NOT GET MORE child support because she is not working. The system should provide Betty with some type of incentive to seek employment and better her life, for her son and herself.