Kat must like her very much, to defend such shittiness. Maybe she's feeling guilty about letting them live there in the first place, and angry with herself, she takes it out on you.
(That's me trying to find a possible positive in anything; better misguided guilt than "she's just a bitch who thinks you're irrational for not wanting to fail drug tests and for your house to not have dumbass stoners in it")