Yea I assume poachers shoot elephants, not trap them. So they only want to shoot ones with visible tusks.

Edit: I found an article[0] describing some poaching methods and apparently they do use traps! Including snares and a spiked board. But that it appears homemade shotguns and rifles are the most common method.

[0] http://www.open-earth.org/document/natureR_main.php?natureId...