Is it rude not to respond to someone unless they say "Excuse me" or "please and thank you?"

My cousin (female) is strict. Fun loving mother but strict. She expects her children to use their manners. She was fed up with them not using it and being demanding. So one day the children were like, "Mom let us have a cookie!" She didn't respond and kept doing what do was doing. They kept asking and asking. Then they finally said, "Excuse me mom. May we please have a cookie?" Then she said, "Oh yes, you may have a cookie."
One of my little cousins was being nasty to her so my mom ignored her. It drove her crazy. I was like, "That is a little harsh" and she said, "Whatever she needs to learn. I'm not going to tollerate disrespect and my daughter being nasty to me. I'm not going to talk to her if she is going be a nasty brat to me. She is sweet if she wasn't such a stinker. I'm teaching my kids to be polite and nobody will respond to their rudeness. I'm not having it. Someone is disrespectful, DONE." WOW she is tough. And so blunt. Why are parents so brutally blunt and less forgiving?