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I think what they were probably going for was the idea that a full grapple involves grabbing and wrestling and restraints and such, which arguably takes a little bit longer than the second or so in your round at the end of a charge.
It may also require a bit more attention than you could manage while running headlong at someone--your momentum probably isn't as helpful as it would be for anything that involves a good swing at the end (the sunder, disarm, trip--and of course bull rush and Overrun are all about the momentum), so the bonus on your grapple attempt wouldn't make a huge amount of sense.
If you're thinking a charge and tackle someone to the floor, that could just as easily be a trip, where you attempt to grapple them the next round.
The grab 2 people with penalties I can see being a thing (although obviously you'll need someone who's insanely good at grappling to offset the penalties you'd rationally have) and banging their heads together could just be a flavorful way to describe an unarmed strike against two separate opponents (since generally you don't hold on to the people after banging them together).
That said, all of this is fairly logical and I'm certainly not saying that your argument isn't sound. In fact my primary gripe about grapple being standard action only is not being able to use it as an AoO--I feel you should be allowed to try to grab someone when they're doing something you don't want them to do. But devs decided it made more sense this way I guess.