No one was allowed to reenter the match and officially eliminate anyone else in 2005 either. Add to that that when Randy Savage launched himself over the top rope in '92, he was allowed back in, yet in '97 some random luchadore leaps off the top to the outside without even thinking and the referees tell him he's been officially eliminated thanks to his own mistake.
To be fair though, WWE is like this with nearly all of their rules. It isn't exclusive to the Rumble match. They alternate and change which rules are in place and what aren't depending on whether it's convenient for the present story they want to tell. It's kinda annoying, but I've just learned to live it because WWE.
I had no issue with how Brock was eliminated, but I don't think getting taken out by surprise would have made him look like a tool. In this case, the surprise elimination would have occurred by Bray in the heat moment while Brock was fighting off a bunch of guys he had already eliminated from the match, so it would have been fine. Goldberg was taken out by surprise in 2004 in a similar manner, he didn't look like a geek.