@Riot is it normal that I win a 4v5 and the guy afking gets the win anyway?

I dunno how i should feel. I get afks quite a lot. I never know whats going on with them penalty wise or what is done by riot to stop this from happening in term of consequences. But i won a 4v5 just now and that afker who rages quit at 4 min cause he gave 2 kills early got the win and the LP...may i know why?