If you are dominating a fight, and your opponent decides to get dangerous with his fouling, the smart move is to end the fight as soon as possible if the opportunity presents itself. If they asked Garcia if he was okay, and he said no, then he made the smart move. You cannot allow your opponent to cheat in a way that endangers your health just because you're beating his ass.
The first headbutt broke his nose, maybe the next one breaks his orbital bone. Maybe he gets permanent eye damage, from a guy who is so far behind on the cards that he resorts to cheating. Fuck all that.
Mikey Garcia is a guy who has attended brain research conferences. He is very aware of the dangers he faces as a boxer, and I doubt he's going to allow himself to be seriously injured by some outclassed brawler who just wants to draw some blood with a few headbutts before he inevitably loses the fight.