Yeah, I thought it was a bit odd, too. But the Rebel Legion accepts Jawas, Tuskens, Wampas, Clones, and Mara Jade as costumes that are also accepted in the 501st. I think they did it to pad their costuming repetoire, myself. They see the 501st with denizens, so they pulled out the "creatures" section, which consists of Jawas, Tuskens, for the most part; and Wampas.
I agree with you, I never saw any good guy Tuskens. Unless you want to consider that A`sharad Hett was a Tusken, as was the other Hett and Tahiri Veila (from the EU). But those would have been face characters and would have been under different catagories. The only other consideration would have been the Tusken tribe that Anakin befriended when he was stuck out in the desert, all night long, due to a storm. This happened just before he met Qui-Gon and Obi-Wan - referenced by the novelization of The Phantom Menace.
Who knows, maybe the Rebel Legion is throwing the Tuskens a nod due to Anakin going all emo hack 'n slash on them, for the death of his mother.
- Bill