Good question!
I think this is one of those questions that needs a more official answer from Elvis or another officer, but here's my take.
First of all, he has to be under threat. That means you are shooting at him (were you? I assume you were) or you're on his radar, which you weren't. In a scrim, you can't get out and attack unless you are fired at. In a public server we're more lenient, you just have to be "under threat" and he obviously knew a loaded rocket was behind him, so I think he justifiably felt under threat.
But was he camping? If you're correct that he was 3/4 of a nav distance ahead, then he should have time to get the nav and get back out before you enter (unless he drives in/out of the bases like I do lol). Whether it was camping depends on just exactly how far ahead he was, and keeping in mind he can't see behind, if you're shooting at him he doesn't know how far away you are. "Not on the radar" doesn't mean much - off the radar isn't very far away when a rocket hog is behind you. If rockets are whizzing by he probably thinks you're very close.
Assuming he was under threat (and yes he probably was), and not waiting too long for you to arrive, then it comes down to: did he pick a clever spot to defend, or did he go too far and, as you say, turn a defensive maneuver into an unfair advantage?
The "hide and wait, then attack" applies to walkers hiding behind a rock and then nading/rocketing when a hog is unaware they are there and has no real shot at them. If you are going to hide, then you can't wait and then suddenly attack. In this case he's not really hiding - you know he's in there. But it is similar in that he's in a defensive position where you can't get to him, and then he turns on the attack and you have no chance.
He's happened to pick a spot where he can fire as you enter, and you'll be dead before the back half of the hog is even in the base, so your gunner has no chance. If he'd come around a corner (say on Infinity, Timberland, Danger Canyon) and ambushed you, at least he'd be in the open where you could have a shot at him if your gunner was paying attention. Even at the bottom of a big jump like on Death Island or Timberland, if he was waiting at the bottom I'd say that was totally fair. This situation seems different, not so fair.
Now if he had been on foot, no hogs, and walked into the base with a rocket launcher, and killed you as you were entering the base, you'd have no chance either, but it would be perfectly legal. Happens all the time. Hmmm.
Or if you were right behind him as he entered the base, I'd have no problem if he jumped out and rocketed you before you could react. It all comes down to just how far ahead he was IMO. If he really was quite a ways ahead of you, then stopped and set up this shot, I'd say it probably went too far.
If you were firing at him, then maybe the answer is "you missed your chance, shoulda hit him!". But yes, I see your point of view and tend to agree it sounds a bit unfair. Did he have the time/opportunity to get his nav and get back out of there? If so I think he should have taken that option. If he felt that he was 100% dead because you were right behind him and he's defensless, then he's justified.
As much as I sympathise with you, and think this may have crossed the line, in a public server, he probably felt justified and the admins were probably right in thinking he was under threat and defending. It's a special situation, not many maps have places where this sort of situation could develop.
Edit: ha, took me so long to type all that in, Wic and JANE answered! Wic seems to have a lot more info that changes things a bit