Without arms all of the Half-Life 2 weapons lie at 1500 polys. Only the left half of the gun is modelled and hence we get the number 3000. Sure the engine can cope with a lot more, but it's nice when all the weapon models you make for an engine lie at the same poly count.
Because all weapon models that Valve have made for the Source Engine lie exactly at 1500 triangles. All of them. They must've done it for a reason, hence most people aim to make it at 1500. I think it's simply the budget that the modeller was given but, regardless, it's the same budget I use.