Horde Warrior Rifleman Now at TinMan

But why does he have his muzzle practically touching the ground? I expect laser/whatever weapons would be the same as gunpowder weapons – some NCO should be by at any moment to tear a few strips off this guy and possibly put him on extra duties…

From the manufacturer blog linked to in the article:

The figure originally had a taller base (that's why the barrel touches the ground) but that went off to get produced in resin by a mold maker and is in limbo. It looks like it will have to go into production this way.
If I can't rectify the situation, then I apologize in advance for being forced to sell a mini that looks stupid.

That answers that, I think. The Greens are stereotypical fictional barbarians anyway, they don't have NCOs and denigrating another warrior's weapon handling skills is a good way to start a duel to the death.

Because Barsoomians despise missile weapons so never fire them anyway

Like fun. Those long guns are the Greens' antiaircraft guns, their answer to being bombed and shelled by the flying warships of the Red Martians. They blow Dejah Thoris' ship out of the sky early in the first novel. Good thing, too – Johnny never would have met her otherwise. Tharks are like Barsoomian Cupids, if Cupid carried a radium rifle.