Attending a playgroup

At one point, she was playing in the ballpit, and an older girl came up to her and started pulling on her shirt.  The girl’s mother saw this, but did nothing.  The girl then jumped into the ballpit, and she was clearly a hazard to Abi because she was trying to swim in the pit, throwing about her legs and arms.  Her mother again did nothing.  What’s wrong with people?  (Abi left the ballpit, and was not hurt.)