Do tusk poachers usually kill? Seems stupid to kill the golden goose.
Penalties for poaching are generally paid with lead.
Also, I thought most African elephants in managed wildlife areas have their tusks cut by wildlife officials to deter poaching and save animal lives. If this were the case, perhaps attraction in mating also no longer includes tusks as a desirable characteristic since few/no individuals possess it any longer.
As to potentially giving your position away due to gunfire, the savannah is a big place and relatively flat. You can be hours away from the next human being. You can see them coming from miles away. And it makes it difficult to tell where sounds are coming from. So while you may hear the shot, you're not going to be able to tell exactly which direction it came from.
Also, you only need to be far enough away to not be caught.
AK-47s and their ammunition are abundant in many parts of the world. I don't think the same is true for dart guns for large sedative darts.
EDIT: I don't know what weapons poachers are using, so AK-47 is just the first thing that came to mind. They maybe use higher calibre weapons?
But your point is valid. Ammunition is easier to acquire and/or make yourself.
You can heave a cannonball at an elephant with a potato gun, but it won't do much. .45-70 Government isn't a Big 5 cartridge despite being almost 12 mm across because of its antique origins and low chamber pressures.
A tiny FMJ high velocity round like a 5.7 wouldn't do much despite having the penetrating power of being essentially a supersonic "needle".
.338 Lapua requires a 5400+ J load to legally hunt B5 in parts of Africa. Otherwise, a 12.7x108 or .50 BMG would do the trick. Anti-matériel rifles are a lot easier to acquire than useless, antique elephant guns. Barrett.net
KE = 0.5 x m x v^2.
Unlike a goose that keeps laying eggs, tusks don't grow back. They are actually teeth.
And even if poachers did remove tusks humanely. Having no tusks or having infected tusk wounds would be a big disadvantage.