30% of Mortgages are Underwater --- Leading to Foreclosure

I came across this article this morning on CNN Money talking about how many mortgages are underwater.  While I appreciate that this isn't a great thing, I'd like to understand how it's second only to unaffordable payments in the causes of foreclosure.  

The article makes no reference to how this leads to foreclosure... is it due to not being able to sell the home and not being able to make the payments?  No that would be the number cause of foreclosure.  I'm assuming that this is due to people walking away from their homes because they owe more than their worth.  

This isn't exactly some new phenomena.   It happens everyday at New Car Dealers across the country.  Why is it somehow socially acceptable for home buyers to duck their responsibility because the home has depreciated?  Also, who's to say that five years from now the housing market isn't going to be in a better place?

It infuriates me everytime I see an article about a stratigic foreclosure, or other mention of some deadbeat giving their house back to the bank.   I have no sympathy for your bad financial planning or luck.  Lie in the bed you've made and be a responsible adult.  

Where am I looking at this wrong?