Because the LOI is a binding contract. And he also has not enrolled yet, so he can't "enter the Portal". He's in a gray area where he is under contract and cannot talk to other schools.
If the school releases him from his LOI, he can sign another LOI (or just enroll elsewhere). If the school forces him to honor the LOI, and he enrolls, then the school uses an IC, even if the kid hits the Portal 24 hours later. So if you feel strongly that a kid is unhappy, it just makes common sense to release him and save the IC. Yes, it's a short term "we lost a Top 100 kid" hit, but UGa is the defending national champs and have a top class with or without one particular kid.