When I DM I expect clerics and paladins to follow their god's teachings. The paladin choose to follow a god who puts strong emphasis on protecting the weak and never turning your back on wrong doing. So if he just walked way washing his hands of what the party had discovered there would have been some kind of consequence.
One of my house rules for clerics is that they must be the same alignment as their god.
And I think, thus is born Pemerton's concerns over railroading. If the player doesn't do what the DM wants him to do, the DM punishes the PC. That's pretty much by definition railroading.
Now, most player will totally accept it as a condition of playing a class with a strong alignment component - they've probably got a concept built in that will hopefully follow along with the strictures outlined by the class. Or, to put it another way, if you play a 2e Paladin, don't bitch that you can't slaughter peasants.
I think, though, Pemerton is simply pointing to a different style of game where the goal of play has nothing to do with the standard rewards of play - accrual of power/wealth in the fictional setting, or even exploring the setting - but rather a thematic approach to the game that is completely separate from those rewards.
If my goal during play is to undertake an examination of the moral implications of slavery (for example), and the DM flat out rules that slavery is evil, then, well, my play just went out the window. We know the answer and all of our actions will flow from that answer that has been handed down by the DM. My "good" character won't do things that are flat out evil, because, well, I'm playing a good character.
I'm not convinced railroading is the right term. But, I can see where Pemerton is going with this. If the players (and you really have to have the whole group on board with this sort of thing) want to explore a thematic (rather than setting or game) concept, having the DM flat out state that X=Y means that the exploration is finished. We know that slavery is evil because the DM told us so. There are no more moral implications that can be explored in this particular game.
Going back to the angels vs elementals example. If the DM has flat out stated that elementals are evil, and we're playing in Elf Witch's game, any good aligned cleric or paladin would pretty much have to help the angels, otherwise, they get whacked with the alignment punishment stick.
Now, that all being said, if the players are on board with a more traditional style game, then obviously there's no problem. The players can decide whatever they want based on their own priorities. But, if the goal of play is to examine whether or not elementals really are evil, then, if the DM flat out states "Elementals are Evil" then play pretty much loses a lot of meaning.