It's a pretty big fraud. A one-day park hopper is $125 whereas the difference between a 4 and 5-day park hopper is $15. So $110 of 'theft' per person, moreso if they 'rent' out more days. Disneyland can't sell the first day for $15 to anyone, obviously. The more people that skirt the rules, the more the honest people have to pay to make up for that lost profit. It's not much different than shoplifting.
If it's Disney's policy to not allow people to let someone else use the multi-day ticket, then I would not do it because I follow rules. However, I'm not sure I agree with equating it to shoplifting. If I buy a six pack of soda on sale and I only drink 5 cans and let someone else have the other can, even though they would have had to pay full price...is that similar to shoplifting?
Though I can see your point in the case of the last day of a ticket being given to someone who otherwise would not have gone at all. But when someone buys a used ticket, that's not what's happening.
I'm not sure what the difference is. Are you talking about giving the ticket away (for free) for someone to use the unused days...vs...selling the unused days for a profit (like scalping tickets)?