Completely correct ruling.
Imagine you raised to 3500 as before, but now all-in guy had 3501. Would it be fair for you to be allowed to go all-in just because someone raised by a single dollar? If so, you would effectively be allowed to bet twice. So the arbitrary standard is that the all-in must be at least a full-size legal raise or the action is not reopened for you.
WPT rules are not official (far as I know, WPT has few rules that relate to actual play--they leave it up to the host casinos to run things). WSOP rules are not official, except at the World Series. Closest thing to a standard for tournament rules is the Tournament Director's Association rules. WSOP basically cut/pastes from TDA rules into their own, as do many casinos nowadays. From
http://www.pokertda.com/rules.pdf we have:
[ QUOTE ]
10. Raise Requirements
If a player puts in a raise of 50% or more of the previous bet but less than the minimum raise, he or she will be required to make a full raise. The raise will be exactly the minimum raise allowed. In no-limit and pot limit, an all-in bet of less than a full raise does not reopen the betting to a player who has already acted.
[/ QUOTE ]
Last sentence is the important one.
Google for Robert's Rules of Poker and you'll find this topic covered within, as well, for cash games.
Stack size awareness is really important sometimes. Often if I think I may induce a short-stack behind me to go all-in by betting, but there is another player in the pot and I have a strong but vulnerable hand, I will size my bet such that an all-in by the short stack will just reopen the betting. So if he has $80, I'll open bet $40. Short-stack all-ins. Next guy calls $80. Now I can push. If I'd bet $45, I'd be stuck just calling the all-in.
Applied to your example, if you'd raised from 1000 to 2600, say (+1600), and the all-in guy had pushed for his 4300 (+1700 more), you'd be free to reraise. But you'd have to decide whether that was likely enough to happen to offset the increased chances of having unwanted callers with AK if you raise to 2600 instead of 3500.