I don't get this. I have a very high regard for correct etiquette in poker, but as far as I can tell, whether you're playing for a main pot or a side pot, you're playing poker, for money, against someone else. The rules haven't changed, the prize hasn't changed, you started the hand hoping to take everyone else's money - what's changed?

I would be genuinely grateful if someone could explain to me WHY it's considered by some to be bad etiquette to fight just as hard for the side pot as for the main.