За что конкретно Уизли записали в "предатели крови", точно неизвестно (по крайней мере, не вспомню цитаты из канона)

A minority of these families publicly deplored their inclusion on the list, declaring that their ancestors certainly included Muggles, a fact of which they were not ashamed. Most vocally indignant was the numerous Weasley family, which, in spite of its connections with almost every old wizarding family in Britain, was proud of its ancestral ties to many interesting Muggles. Their protests earned these families the opprobrium of advocates of the pure-blood doctrine, and the epithet ‘blood traitor’.