Tahnk you @CartoonFan311 and @Paul The 57th Engine Just because Percy is a weasley doesn't mean he is fully innocent as the reasons explained above
Note : Percy actually had a chance of being in Slytherin since he was ambitious , clever , a leader (leadership) , self-reliant etc . I think the only reason Percy was not in Slytherin is because that would probably mess up some things that happened in the series .