How could Greece-related worries drive US stocks down about 2% in a day? Major US stock indices are down ~2% on Jun 29, and it's mostly associated with investor's worries related to the Greece crisis.

I understand about possible "chain reaction" and that a lot of companies may be exposed indirectly to Greece debt.

But given that, it seems silly that investors think that because of this average US companies lose 2% in values. I mean, Greece is relatively a small country and it doesn't have a very close link to the US economy (unlike, say, Canada). Even if all Greek government and banking bonds evaporate, the impact of all those would still be way less than 2% of an average US company, right?
Because Greece sets a precedent for Italy, Spain and Portugal (and others). 
And a default of Italy (and Greece) might will cause financial difficulties in France. 


If the interest rates for Italy and Portugal rise (they do now), then those countries are even more unlikely to repay their debt. 
And meanwhile, somebody  has to write them off.

Greece in itselfs really is kind of unimportant (except for France). 
That the debt really cannot be repaid is at this point kinda obvious to everyone. And while a write-off of 80*109 € (or part thereof) is painful for Germany (and others), it's simply not the size that can seriously jeopardize an economy of 8 times the polulation and 15 times the GDP of Greece. The risks of political fallout in Germany are another story (and the problem of Merkel).

Greece mainly is kind of an experimental laboratory of things to come (Syriza in Greece = Podemos in Spain, Italy => ? ...)

And the experiment seems to spell disaster. 
Meanwhile, nobody likes to be the labrat. 

As always, the stock-exchange is the place where the expectations of the future generate the prices of today (together with the panic of the moment).