The Ewoks had never seen anything like a shield generator before. They had no idea what it was, and no reason to think it was bad, until their new Rebel friends said they should attack it.
Going back to the beginning of this thread, the song 'Ewok Gospel' (which I happen to think is awesome and hilarious) was never intended to be taken seriously. It's just meant to be fun. I love Ewoks, they're adorable and brave, but I have to admit that in real life it's unlikely that they could have routed the Empire like they did in the movie. I think most people realize this, even the people who created 'Ewok Gospel'.
Okay, so these little teddy bears don't win the prize for being the most kick-butt military group ever. A bicycle doesn't win the prize for being the fastest mode of ground transportation, but nobody cares. Crazy speed isn't the point of a bicycle, and military might isn't the point of Ewoks.
Besides, this is a movie, after all. You have to suspend disbelief and just go with it. You can find unbelievable things in just about every piece of fiction. To cite a popular example, in most versions of the Superman story, a different hairdo and a pair of glasses are enough of a disguise to fool Lois Lane for a long time, even though, in most versions, she knows both Clark Kent and Superman well. This kind of thing can ruin your enjoyment of almost any story, if you let it.
I don't think victorious Ewoks are the most far-fetched thing in Star Wars anyway. What about Anakin creating an extremely complex machine like C-3PO from any old thing he could get his hands on? Not to mention C-3PO's incredibly sophisticated programming. How does a nine-year-old with no training and no resources do that?
It doesn't really matter. You can't get hung up on this stuff. You just have to let go and enjoy the story.