The villagers yell at Beetle because they are angry at the Midwife.  Since they need the Midwife's services, they take out their ire on her little apprentice, Beetle.

The Midwife is a greedy woman who will only provide her help to those who can pay.  She spends a good amount of her time haggling with the villagers over her fees, and has been known at least twice to have refused to come to the aid of mothers in actual labor who could not afford to give "a silver penny or a length of newly woven cloth or the best layer in the hen house" in return for her services.  These poor women had been forced to bear their children on their own, with only their willing but unskilled neighbors to help.  Needless to say, this did not endear the Midwife to the people, but, as she was the only Midwife in the village, they did not want to get on her bad side, lest she refuse to come to their aid if their time of need should arise.  Instead of making their frustration known to the Midwife herself, the people began to take it out on her  apprentice, the homeless waif, Beetle, who was "needed by no one".  Beetle endured their taunts and anger stoically, refusing to respond, and complaining only to her faithful cat (Chapter 3).