Why do some people call assertive parents "mean?"

My parents don't take shit from us. They made us pay for the consequences and make us remember what we did. Our late mom was difficult to please. She says sorry doesn't cut it and she bluntly told us if we did something like that again the punishemnt will be harsher. She was a from believer in tough love and so is my daddy.
She was very assertive and when we push her, she called us out. My dad is very assertive too.
I called my parents meanies. My mom smiled and said, "We don't mind being called that." And walked away.
Dad said, "You'll thank us one day." I screamed so loud!!!! My mom ptopped my mouth and told me to stop screaming. I have a stronger relationship with my dad. I did with mom too but she passed away almost two months ago.