Originally I thought this was completely stupid, and in no way anything that could benefit Christian at all. After I've had a couple days to rethink the situation, I've completely changed my mind and opinion.
I think that this is basically what W.W.E. thinks of T.N.A., Christian was a Heavyweight Champion in that company. However, he was nothing more than a mid-card attraction in W.W.E. So upon his return, he's remade a bigger name for himself as a Heavyweight somewhere else, yet that still isn't what the W.W.E. claims as a Heavyweight themselves. Thus, he's E.C.W. bound.
On top of that, let's run down the two glaring facts on Christian returning to either Raw or Smackdown.
Raw: If Christian would've returned to Raw, what would his position be? He's clearly not, and likely never will be a bigger name and attraction than the likes of John Cena or Shawn Michaels. So it's highly unlikely he'd be featured in the Main Event over them, least of all Cena. Thus.. Raw isn't a good option for him.
Smackdown: While if he returned to Smackdown, everyone could get their fix on a feud that pretty much ended in 2001, the true fact is.. he'd always be second rate to guys like Jeff Hardy and Edge. He wouldn't be seen as bigger than either of them, especially while feuding "under" them. If he were thrust into a feud, reforming a team with either of them.. he wouldn't be seen as the Main individual in the team, so much as a "supporting cast member". Thus.. Smackdown isn't a good option for him.
Now we have E.C.W...
E.C.W.: Since Matt Hardy turned heel, and left, the brand has been without a top face to lead the Brand. Since Jack Swagger became Heavyweight Champion, and they're clearly focusing the brand on being a more important thing than it ever has been. They obviously need someone with a big enough name, to help lead it. Enter Christian.
Christian isn't being "doomed" to the 3rd Brand, he's being handed the keys to an entire Brand in which he can be the Main focus on. By returning, he instantly became a bigger name on the brand than the likes of Finlay, Boogeyman, and any other face you could think of on that brand. By defeating Swagger, regardless of how, he is now instantly thrust into the role of the next Challenger.. and by that, he'll help make Swagger an even bigger name.
It's a win, win, win situation. It helps Christian to readjust to coming back to the company that helped make him. It helps Swagger to become a bigger name, fueding with more important people. And it attracts all types of fans in general, to have to pay attention to this brand.. if they want to see Christian.