They did it for “an abundance of caution”
Can you imagine what would have happened if they hadn’t grounded the planes and a few of them would have crashed?
It’s easy to have 20-20 hindside vision.
Also, for some who think a cloud is the same as the ash: Some articles said the ash contained SO2.
“Pilots can never fly through clouds of volcanic ash because these bits of pulverized rock and glass can melt in aircraft engines, causing power loss. The ash can damage aircraft electronic, hydraulic, ventilation, and data systems. Sulfur dioxide, another product of volcanoes also carried within the ash clouds following an eruption, is corrosive to aircraft. “
http://www.ens-newswire.com/ens/apr2010/2010-04-15-02.html