Because she deserved to go to prison. My questions are, why was the sentence so short to begin with? Why was all but 6 months suspended?
Putting bodily fluids into any food or drink intended for consumption by another person is a horrible thing! Diseases are transmitted by bodily fluids. The twisted individual in this case should have been sentenced to far more than 3 years in prison, and sued into bankruptcy by her victim.
The customer could have contracted viral infections from her (no telling what a person with disgusting habits like this has for a medical history). Hepatitis (several strains), AIDS ... who knows what this pig caused the customer to be exposed to.
To anyone that considers something like this a joke ... I hope you are never on the receiving end of a "joke" like this.