In my opinion, the unwritten rule about checking it down to have two chances of knocking out the at risk player only applies when there is one single pot, ie the all in guy pushed his chips in and got two flat callers, so no side pot was created. In your scenario, a side pot had been created, and you therefore had every right to continue to play poker to try and claim that side pot. If you win the main pot as well, then so be it, but at the point that you made your bet post flop, your intention was to claim the side pot.
I do get a little peeved though when someone is all in, and you get someone making a bluff with no side pot in play. They are effectively using their chips to bluff to win an empty pot - and that is stupid play. In that situation, I believe the unwritten rule should be stuck to.
The third scenario is where you have something like JJ, and the flop comes 10 high, with a player all in. It is then a reasonable play to bet and push the third player out to protect your holding..... your JJ might be the best hand in play at that moment, but with a very real chance of losing out to the third player on the turn or river.