[color=darkred:b2arwfoc]Good question!
Good and evil can exist without the existance of Satan. I don’t know about you but when I committ a sin I do just fine on my own without any help from Satan. 
As to why he would let him remain, I can only imagine there is a reason for it but I can think of any at the moment.
The same question can be asked of people of Hell, no?[/color:b2arwfoc]