In theory (again I haven't done this yet) you could really utilize this to get some pretty awesome LLs.
Lets say you were only going for 4 days at this end of this month (1/28-1/31). In theory you could buy the 14 day ticket (18 day valid window) which could be valid for 1/14-1/31... since you are offsite you could book all your LLs 3 days before your ticket is valid on 1/11.
My guess is that it's just not worth the legal/logistical hassle of closing the loophole with 3rd party sellers, so they don't.
In the most extreme case, someone would buy this ticket for about 545pounds or $735.79... Assuming the 3rd party seller keeps 15% of the sale, Disney is making $624.42. The most Disney could make from this person is an annual pass at $1,967.06 (Incredipass + Photo Download + Waterparks). So they lose out on a maximum $1,342.64 per American.
So to even lose $1 million on this, 745 Americans would have to make this decision...
Considering this forum is some of the most Disney obsessed fans I know, and most people on here don't/didn't know about it, I would be surprised if the numbers are super high.
On the flip side, because this deal exists... I'm considering giving Disney $345 in ticket sales plus additional LL, parking, food, and souvenirs because it feels like a better deal and might draw me to the park an extra day or 2 (or stay longer some days because I can hop).