Yeah, games shouldn't even start if someone never connects. Just kick everyone back out to lobby and count it as a dodge for the person who didn't connect.
But you can't have loss prevented for afks... Even if they afk from the start. Let's look at this exploitable example:
You are in champ select and things aren't going well. Players are troll picking and fighting and you just want to dodge... but... that lp and time penalty is so annoying. So you stick it out and the game starts.
Then you just minimize the game and never play. Oh look! Your team surrenders at 20 from being slaughtered in constant 4v5s and... LOSS PREVENTED! Woohoo, no lp lost, and only 20 minutes penalty instead of 30+
Yeah... so loss prevention on afk does not work.
You could say "well just punish the afker, don't give them loss prevented but give the rest of the team a loss prevented"
But... I can see a way to abuse this as well.
You and a friend duo q... and then one of you goes AFK from the start. At this point it's a 4v5, but that's fine. Odds are, someone on the enemy team is going to feed or troll or who knows... So maybe it's closer to a 4v4.
Either way, your 4 man team plays and if they win, awesome, they all get LP and IP and a win. If they lose, that's cool too, because... LOSS PREVENTED. So it's essentially a way that you could play the game and never ever lose a game, but there is a possibility that eventually you could win some games. So you could literally game the system to never give you a loss... Ever.