But the droids aren't the only element of this episode. We also learn more of Zeb's backstory: he saw these weapons in action when the Empire used them on his species. The event has clearly scarred him, causing him to drop his usual rough exterior and ask whether Kanan should sell these weapons at all. It's a fantastic character moment, but it's sadly used to prop up an unnecessary plot twist: Agent Kallus (who hunted Ghost in "Spark of Rebellion") was the one that ordered Stormtroopers to use these weapons on Zeb's species. You know, because the Empire can't be villains unless they're evil on every conceivable level, even borderline coincidental ones.
While we're at it, Zeb takes point on the episode's impressive melee-combat sequences... possibly too impressive. Despite being built like the Ghost's version of Chewbacca, he's insanely agile, dodging Stormtrooper blaster fire and diving into acrobatic melee combat. It's well-presented, but the Ghost already has two Force-sensitive characters, including a former Jedi. Why don't they break out lightsabers for that style of fighting, so Zeb can dish out a tank-based approach? It's not like Kanan needs to hide it, since he already revealed himself to Kallus last episode.
Not that this doesn't make for a good episode, it's a lot of fun. But combined, "Droids in Distress" really has a one-step-forward-one-step-back tone; every great idea is countered by another that's silly, unnecessary, or doesn't quite work. To top it off, trying to pack everything into a half-hour timeslot makes the episode feel rushed, so that reveal regarding Zeb's species only takes five seconds before we move on to something else. The concepts here probably would be better served with another double-length episode, or failing that, at least been trimmed down to a more manageable focus.
It's also worth mentioning that Rebels is pushing its "criminals-with-hearts-of-gold" premise in some strange directions for a children's show. Despite being heroes, these characters are way too comfortable with the idea of selling overpowered weapons, especially to the individual who probably betrayed them last episode. It's especially bizarre coming from Kanan who, as a Jedi, should be well-aware of the consequences of these actions. But not only does he enthusiastically pitch the job to his crew, he hand-waves Zeb's concerns away with "as long as the Empire doesn't have them". I'm sure future stories will address this episode's fallout, but right now there seem to be some conflicting motivations happening here.
Bottom Line: "Droids in Distress" is a solid Rebels outing overall, but tends to undercut every great idea it puts forward. Characters are fleshed out in some interesting, if contradictory directions, and very exciting seeds have been laid for future episodes. But even cameos from C-3P0 and R2-D2 don't make this chapter better than average.
Recommendation: This episode is probably setting the stage for impressive follow-up episodes, but I sense that it probably won't be the most memorable episode of the season.