Student or Learner
If someone throws biscuits after bad dogs, does it mean that they give something to whom not derserving it?
Well, I would agree that "casting pearls before swine" means that you are giving good things to people who don't appreciate/deserve them, but "biscuits after dogs" one sounds more like "throwing good money after bad." That is, after something starts going unrecoverably wrong, don't spend more resources trying to fix it.
[not a teacher]