It's quite sequitur, because the existence of the tsunami means you only have acquired evidence that the reactors are dangerous in massive, already overwhelmingly dangerous situations. If a tornado destroys all the houses on your block, causing your car to roll out of your (destroyed) garage and run over a mailbox, no one goes around talking about how this proves the dangers of cars. (I exaggerate the comparison here, but you get the point.)

Yes, obviously the cost of these ultra-rare events needs to be accounted for in doing a cost-benefit analysis. But the billions of dollars it will cost to clean up Fukishima (which, again, are mostly unnecessary) is tiny compared to the total cost of nuclear power, or any power source, on a global scale. There are ~500 nuclear plants worldwide, each costing ~$5 billion each. This singular event is just plain not important.

The fact that it's "avoidable" is only useful on the scale of improving particular reactors or particular regulatory agencies. With regard to choosing sources of power, it really doesn't matter except to make nuclear more attractive. (If some nuclear power risks are avoidable, and therefore improvable, in a way that the costs of renewables cannot be improved, this is a point in nuclear's favor.)

The question of which types of power society should invest in is a complex one, requiring the careful weighing of various risks, technological uncertainties, and ethical questions. I certainly don't know the answer. But what I do know is that the pittance of money lost in this disaster does not play an appreciable role in that calculation, except insofar as it gives ammunition for some people to fearmonger.