Well I can see where the OP is coming from, but your comment about choosing to have two children and not choosing to have twins puts me firmly on your side, but possibly only because I have twins too. I can see why non-twin parents wouldn't necessarily feel that way.
The things that gets me about that post was that it should really be a non-issue. The thought about school trips and having to fork out double each time one comes up really scares me even now, and my two are only 2 (next week). I've just had to fork out for their uniform for nursery, which they start in 2 weeks, and that was painful enough. If was offered subsidy from the school like this I would be incredibly grateful and thank them from the bottom of my heart, but I would have had the sense to keep my mouth shut, for my DTs sake if nothing else (no child wants to be made to feel different or 'poor'). It should have been kept private, and then there wouldn't have been anything to post about.