I don't think it has ever specifically been the case, but it may look like it in some cases, because many companies bought content on a contract basis. So, for example, if you look closely, you'll notice that the early Doctor Occult stories (Siegel and Shuster) have a big gaping hole in the middle. Where do you read what happened? In the "Doctor Mystic" strip in the Centaur books, available on this site--DC's check didn't clear (or something), so Siegel sold those episodes elsewhere. I'm sure there are other instances of this happening, though I don't know of any personally.
But remember, the STORY carries a copyright, not the character, except to the extent that the character is part of the story. So, as long as you buy the story from whoever owns the copyright and your "competition" doesn't hold a trademark, you can publish whatever you like, starring whatever characters you want. There's nothing wrong with or illegal about that. You just can't derive from Intellectual Property owned by someone else (well, you can, but you run the risk of a lawsuit, of course).
Of course, you might also be referring to the fact that, in books like All-Star Squadron, Roy Thomas created several characters that were heavily based on obscure Golden Agers from other companies (the Phantom of the Fair being the most obvious example), but he also did that later for the Invaders series, as I recall--the latter book having a team with the Human Meteor, Spider Queen, and so forth. That's no different than what the Superpowers series is doing, and I'm sure is its inspiration.
Or maybe neither of those cases is what you mean.