The thing is human thought is remarkably simple and predictable! All the writers are guys with average or slightly above average intelligence who followed what they thought was a path leading to an intelligent sci-fi story!

First step: Here's an idea for a sci-fi movie, guy transfers his mind to a new body.

Next step: Why does it have to be a human body why can't it be an alien body.

Next step: Q - Why would a man need to be in an alien body?

Next step: A - Because he's trying to live in an environment a human can't survive in.

Next step: This planet must be as unlike Earth s possible.

Next step: A gas giant like Jupiter will do, Pluto is just a small cold Earth!

Next step: He can't just go there and do the job, he needs an arc.

Next step: He goes there and likes it there and wants to stay in his new body!

Next step: Yeah the future Earth he came from is a dump!

Next step: He meets a girl, they fall in love and it's his new home.

Next step: But there has to be something wrong with his old body.

Next step: Maybe he's got glasses and really bad acne!

Next step: So do most of the audience!

Next step: No he's in a wheelchair!

Next step: Ok, now all I need are names and creature designs.

The creature designs are really a question of individual tastes, no-one copied anyone they just each went through the motions. (Let's leave the conspiracy theories for the conspiracy nuts!)  :D

As for Ellison, his behaviour, though disgusting and dishonourable are logical, he can't sue Cameron because he has no Avatar-like story to claim copyright breach (that I know of). But let's say that he did,Cameron can hire bigger badder lawyers than Ellison can at this point in time. Ellison can take on the director and writer of a small movie no-one has heard of and since it's not a remake, no-one can say they're following a story that was copyrighted long ago!

It's basically the school bully picking on the kid he thinks has money, whom he thinks he can take on!  :(