How is it terrorism when they kill soldiers?

[Evil_Maniac From Mars]

They're not trying to terrorize the population just to terrorize them, they're doing it to ensure loyalty.

This sentence alone proved my point. The point is that they are doing it. It doesn't matter why - terrorism is a tactic, not an end.

If I was an Afghan I would target soldiers because that's the best way to let the country pull out the troops.

You are a Taliban fighter, not an ordinary Afghan.