"Terrorism" became an overloaded term once we coined the term "War on Terror".
While this is an act of terrorism for sure, this is not the Terrorism that originates from those we are supposedly fighting in the War on Terror, therefore we should not raise our country's alertness level, have reason to panic, or mobilize our armed forces in response.
I tend to agree, but I'm a bit more cynical. These days governments use the word "terrorism" to describe acts against which they would like to retaliate with the backing of their citizens. The term has become a trigger word for prompting a particular reaction from the people. There's nobody to retaliate against here and using the word terrorism in this case would simply water down the trigger effect of the word in the future when the U.S. government wants to prompt a specific response from Americans.
While this is an act of terrorism for sure, this is not the Terrorism that originates from those we are supposedly fighting in the War on Terror, therefore we should not raise our country's alertness level, have reason to panic, or mobilize our armed forces in response.
(my 2 cents)