Imagine having to drink coffee. Its pretty terrifying, especially for a Brit.

More seriously - the people who own that property were civilians and probably scared. In usa terrorism is defined currently as "premeditated, politically motivated violence perpetrated against noncombatant targets by subnational groups or clandestine agents". (Title 22 Chapter 38 U.S. Code § 2656f)

The boston tea party was premeditated, politically motivated, the tea was not military (so non combatant) and it was done by a subnational group. Seems to fit to me.