Good Samaritan detained by MRT staff for pressing emergency button to help elderly man - Terence Teoh

The Real Singapore
On tuesday night, I was taking the south-bound MRT train towards Ang Mo Kio.
At Yio Chu Kang station, a woman asked for help in a desperate tone, then pressed the emergency button on the train.
I realised that an elderly man had his arm caught between the train doors. The doors did not re-open after the usual few seconds, and his arm was stuck for about a minute.
When the doors did open, the old man entered the train and was unharmed.
However, an SMRT staff member came and demanded to know who had pressed the button.
When the woman owned up, he asked in frustration: "Why you press the button?"
Later, when the train stopped at Ang Mo Kio station, the woman was detained and further questioned. Thankfully, another man stood up for her.
When is the right time to press the emergency button? If someone gets caught between the train doors, are we supposed to wait until the train starts moving before we press the button?
Perhaps SMRT can clarify the protocol for using the emergency button. Link