The mashup is also a literary tradition that goes all the way back to people like HP Lovecraft. Moore has talked about this at length.
The major thing to understand about BEFORE WATCHMEN is that Alan Moore & Dave Gibbons signed a contract with DC that was an unprecedented boon for creators' rights in comics as they were to get all the rights back in a year. But the contract was designed with a loophole that DC abused to keep the rights away. Since then, they've tried to get Moore to do more WATCHMEN comics by saying, "we'll give you the rights" and he's rightfully told them to **** off. So BEFORE WATCHMEN is essentially DC stealing Moore's characters.
The irony of this is that people are continually demanding comics companies recognize the contributions of creators like Bill Finger, Joe Schuster, Mick Anglo, Jack Kirby, etc. The recent Ghost Rider case got everyone up in arms. And yet, Alan Moore, perhaps the greatest comic writer ever, who has his five-page Green Lantern stories turned into year-long summer events, is totally screwed by DC out of the rights of characters he created and all people say is, "well the League is a rip-off".