It's unsportsmanlike only if the referee decides it is. I'm 90% sure there is no black and white rule in the book and it's up to the referee to decide if it's a foul. There are a lot of things that get left under "unsportsmanlike", which gives the referee a lot of leeway.
If you are running behind an opponent and simply say "leave it" in a really low voice, the referee will probably be hard pressed to give it. If you are running behind an opponent and scream directly into his ear and lunge at him while he's shooting or something, you will get a yellow. In general, these type of unsportsmanlike fouls have more to do with appearances than competitive advantages, in my experience.