yeah, not being able to attack until a turn after you declare war takes away the element of surprise. i mean, that's basically saying, "hey, we're going to attack you...but, um...yeah, you go first."
It's called either having honor or having a sense of decency.
Do you just quit your job out of the blue, or do you give them 2 weeks' notice? The latter lets you have a good reference for the next job, the former doesn't.
If you're about to lose your job, which generates greater resentment: being terminated without warning, or being given a couple weeks to look for a new one?
Who do you have more respect for: the guy in the bell tower who opens fire on everyone without warning, or the guy who phones in a bomb threat so the building can be evacuated first?
How much less dastardly would the Japanese have seemed if their declaration had been delivered at the proper time, and Pearl Harbor had had a few minutes to prepare for attack? Better yet, why did the Japanese bother to try to issue their declaration BEFORE the attack? Why bother to declare at all? Wouldn't we have figured out we were at war when they invaded California?
Do you prefer the bouncer throw you out of the bar without any warning, or to let you know at least once that you're breaking rules you weren't even aware of?
I could go on and on.
The point is, when we're attacked out of the blue, it's seen as evil and cowardly and there are greater repercussions for it than if we know something bad is coming. It used to be a matter of chivalry to declare war and allow the enemy a day or two to gather his forces. Only uncivilized barbarians attacked without warning. The Japanese were reviled by friend and enemy alike during their industrialization/modernization phase last century because they'd attack without warning. Any kind of surprise attack is seen as especially evil.
And there should be an in-game penalty if you choose to behave in a reviled way, just as there are penalties in real life.
And just like nuking everyone at the end of the game doesn't matter, so too will it not matter if you're the biggest kid on the block.