A slave is the tool of her Master or Mistress. A slave should feel privileged that their Master is giving the slave anything to drink, especially something as noble as the piss from the Master or Mistress.

Degrading a slave by being able to urinate on their body or face, or forcing the slave to actually drink their urine is a common theme in BDSM. The unpleasant taste acts as punishment, while at the same time it exerts the control of the Master / Mistress and degrades the slave.