It's cus they think he presented after being in a dominant alpha's presence for so long. That type of plot is pretty common afterall. Where the bets presents as an omega after taking in too many alpha pheromones. And the ml has been acting as a pretty good guy/friend to wooju for years, ofc the fathers are only slightly angry and not beating the shit out of him like they ought to. They don't even know their son was drugged with stuff that was eating away his memories

wow im disappointed by the father’s response. Like what do you mean you know that trash was the one who changed your son who is a victim but just calmly talk to him??