I think the reasoning behind calling them acts of terrorism is that it gives them much greater powers of arrest and interrogation than if they treated them as normal crimes. Morally, they are at least as bad as acts of terrorism (in fact, worse, when you consider that terrorists at least believe that what they are doing is for a "just cause".)
And this vandalism will affect not just Greece, but the whole of Europe, and probably the world.
Although I live on an island off the West coast of Greece, a long way from the current outbreaks (though we did lose 45,000 square metres of forest land earlier in the year), today we awoke to what looked like a foggy British October morning.
My wife was asked by some newly arrived tourists if the sky here is always so strange. She explained that what they were seeing was smoke from the mainland. The sun is a pale, fuzzy disk in a grey and yellow sky...