While I agree on many points of ROTJ's silliness, this scene actually isn't all that nonsensical to me.

We have already met Threepio, an overly emotional droid. This emotional Threepio has been part of Star Wars since the very begining. He is the first character we are introduced to. The idea of a robot being emotional is pretty nutty, but it is part of Star Wars. Even Artoo shows emotion, he often has a little laugh at Threepio's expense, he expresses fear right before being captured by the Jawas. So, it is pretty well established that robots in the SW universe have feelings, and express emotion. Perhaps you could say that they are not really emotional, but are programed to express faux emotion. Either way, same effect.

So, is it really all that unreasonable to think that a droid might not exactly be thrilled about being melted down slowly? Or torn into two pieces? I know for sure the two droids we all know the best would put up a protest very much like the "garbage can" did. Perhaps that power droid wasn't in pain from the burning, but was freaking out with anticipation of being melted down, or perhaps droids are programed to feel pain (just as they are programed to exhibit emotion). In the context of everything we know about the SW universe from the previous two films, to me this certainly isn't one of the worst concieved bits of ROTJ.