The reason is that ONLY the referee can stop the fight!
If you throw in the towel or enter the ring your fighter will be disqualified. Loss by DQ same as if you jumped into the ring and held
the opponents arms down or hit him.
BUT usually the ref takes it as a signal and then stops the fight.
It depends on the particular rules, sometimes only the ref, sometimes the ref and doctor can stop the fight. The ref in question probably didn't like being "told" by a corner man when to end the fight.
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