Disney/Fox has every right to sue. Some responsibility lies with the buyer of things like this. People will always copy. People will always trace. They are all hacks and should be sued, but it will keep happening.
One who irritates me is Roy Lichtenstein. He ripped off images from comic books making the slightest changes and sold these for a fortune. He was smart enough to avoid stealing from the major players: Marvel, Disney, DC, etc... Most of what he stole came from War and Romance comics. What bothers me is that the original artists whose work he ripped off got nothing... and many ended up dirt poor later in life.
As for Bongo's first example... the Simpson's version itself is a direct parody of Peter Blake's cover to the Beatles' Seargent Pepper's. Parody or Satire are protected under Fair Use laws... and I suspect the same applies to the parody of a parody.
Morrison ONLY borrowed the style and composition of the Sgt. Pepper album - and MOST important NO ONE mistook his parody for the Sgt. Cover - important points for parody protection, and imo qualifies as a parody. In the Kaw case - he copied the original Morrison's whole cloth then put x's for eyes.... and from 10 feet away looks exactly like Morrison's work and is often mistaken with it and imo is NOT a parody but a sad commentary on the art world.
BTW the Kaw Sgt.Pepper was a commissioned work by Niko - a prominent Japanese "art" player. So it's possible Kaw didn't even come up with the idea.
There is strong evidence that the anonymous $15million buyer was Justin Bieber.
Here's some thought experiments. What if Matt Groening not Kaws had made the xed out Simpson/Sgt.Pepper work? Would it sell for $15 million? I know his sketches fetch about $500.
What if the original comic book artists had tried breaking into the fine art market with paintings like Liechtenstein. Would they be seen as artworld heroes and innovators - or crass cartooners trying to re-cycle tired old work with a new marketing scheme?