It definitely is a problem, but it would be an even greater problem if the immunity is exploited. If one guy agrees to be the fall guy while 4 others reap the benefits. Especially at the bottom tiers of elo's. There are tons of Gold V players with 0 LP, they have nothing to lose and can move up whenever they choose. It is no loss to them to just sit a game out.
It also can cause unsightly behavior where they taunt the feeding person into afking so they don't get a penalty.
There is also the case where the team with less members in a 4 v 5 actually win. Is it fair to them in this case where they could have just not done anything, yet they get the same rewards as everyone else?
No, its not fair for a game if there is an afk/dc, but in the big picture, the other team probably has a just as likely chance to get an afk too. Have you ever played a game where everyone on your team is doing extremely well? This is a case where the other team is just having the bad luck draw. Fair takes a bit of a different connotation when you start looking away from the individual.
I don't like polls like this because they are incomplete, and a bit warped to try to invoke some kind of response that the creator of the poll enjoys. Its oddly too specific for it to be a poll for riot to base a decision on, even if it get 200k people.