While "officially" this may not be an act of terrorism....I feel the act of planting bombs in public places is terrorism I don't care what the police anywhere on planet Earth call it. Acts such as this cause terror in the hearts and minds of citizens. Just my opinion.
I think the key to the difference in classification comes from the definition of terrorism, which is
"the unofficial or unauthorized use of violence and intimidation
in the pursuit of political aims."
I know there is much controversy and many subtle but important differences in how various countries and organisations define terrorism, but the part everyone seems to agree on is the part in bold above.
The key characteristic of terrorism is that it has a political goal or aim as its core motivation and objective.
Some madman shooting random people because he has a mental illness or just wants to kill people is not committing a terrorist attack because he is not doing what he does for any political aim or goal.
On the other hand, someone who kills a person with a plastic bag is committing an act of terrorism if he killed his victim to serve a political goal.
The method and tools used does not determine whether something is an act of terrorism, but rather the motive behind the act.
There are various tools which are most commonly used by terrorists, such as bombs, but just the use of bombs alone is not enough to constitute terrorism if the attacks have no political motive or aim in mind.