Many economists claim that markets are rational. Like the claims of many economists this one has no relationship with any evidence base at any time when the assumption matters, such as a crisis.
The Asian markets have fallen about 2% as a result of what has happened in Greece. That's significant. But we have no idea if it is rational, or not. How can one tell what is rational when a major currency faces a breakdown? It is not possible, which is why these type of economic claims make no sense in a situation such as that which is faced today.
In that case we do not wish for a rational response from markets today. We want a small and measured one. Such an adjustment may be wholly irrational. We just don't know. But it is what we need because politics has to stump economics on days like this even if, paradoxically, the crisis has been created by politics ignoring the economic reality of Greece for far too long.