We don't talk about Arthur Weasley fighting Lucius Malfoy enough.
In that bookstore, when Lucius insulted Arthur's family (not only that, but Hermione's as well), Arthur snapped. He didn't even bother drawing his wand - he just lunged at Lucius and started swinging.
It was pure instinct. He was defending his family. Like he always has. Because Arthur has spent his whole life defending his bloodline, his wife, and his family from people like Lucius Malfoy.
It all goes back to the early 1900s when the "Pure-Blood Directory" was published. The Weasley family were one of the first families to publicly denounce their inclusion on the list, claiming that their ancestors were tied to many muggles - and proud of it.
This earned them, as well as a few other families (including the Prewett's) who separated themselves from the list and from pureblood ideology as a whole, the title of Blood Traitor.
This means that from a very young age Arthur was made aware of his place in the Wizarding World. He grew up hearing "blood traitor" thrown at him and his family. But he also grew up proud of who he was, where his family stood, and their place in history.
He went on to marry Molly Prewett - another "blood traitor" with a similar background. She, like Arthur, was protective of her family, having known what it was like to grow up being ridiculed.
It's only natural they would raise their family to be the same. Protective, proud, and resolute. And that's exactly how we see the Weasley children in the books. Specifically, how Fred, George, Ron, and Harry react to insults made about their family and friends.
(It also wouldn't be OOC to assume Bill, Charlie, and Ginny did the same. Percy too.)
They grew up seeing Arthur defend them. Not always violently, as it seems rare for his character, but without ever tucking his tail between his legs. And they saw how Molly protected them.
So of course they're like that. Of course Fred, George, and Harry would whoop Draco for insulting Molly. Of course Ron would jump to defend Hermione with his fists, not his wand. Of course Ron would threaten their classmates for doubting Harry.
TLDR; don't mess with the Weasley's.
