Twin-Strike is a terrible example to use if you're advocating changing targets. Twin-Strike, the targets are declared before any attack rolls are made. Only powers with secondary targets permit multiple instances of target declaration, or powers like Blade Cascade that are specifically iterative. Twin-Strike is neither of those, so you follow the standard targeting rules for powers.
Now, unless the countercharger has completely blocked off your routes to the closest square of the original target, you're not prevented from reaching the target. Thusly, your action can proceed as planned, you are able to move to the target's location, and then make your attack. Actions declared do not have their parameters changed mid-step without a specific rule saying they can be. So you -have- to continue forward, even if it's not the most optimal move anymore.
Yes, that is not good for you, tactically speaking. Which is probably why the dragonshield charged you. He used good tactics, and found a flaw in your own tactics.
If your attack routes have all been cut off by the dragonshield, however, then the move portion of your charge action has been negated, and therefore, cannot occur. And as you cannot reach the target of the attack portion of the charge action, that cannot occur either.
Bare in mind, this is all happening within the space of a couple seconds, not the two minutes or so this might take to resolve.
Here's a similiar question.... what if instead of a player charging the kobolds, it was a kobold dragonshield charging the party wizard? And what if the fighter had readied an action to charge if the ds crossed a line, with the intent to -stop- charges on his party? Would you have the same opinion then? Personally I'd allow the player that tactic, because it's -realistic- and -makes sense- and isn't over-powering.
I honestly believe a lot of the opinions here are being colored by the fact it's the monster doing the charging and making life difficult for the PCs, as opposed to the other way around.