I think some of these comments are ridiculously out of proportion. Our family celebrates St Nicholas Day on 6th December. I don't think that parents who don't do this are "incredibly mean" or "depriving" their children.
Christmas can still be magical without believing that Father Christmas is real. My children hang out their stockings with great excitement on Christmas Eve. We "play" Santa in the same way we might play any other make-believe game. It is an entirely adult notion that something can only be magical if children believe it to be factual. Make believe is limited only by a child's imagination, whereas real things is limited by real rules.
Keeping things simple doesn't make it less magical or exciting for children, in fact I think it does the opposite. My children get so much excitement and joy over simple things like advent traditions, nativity scenes, singing carols, decorating the tree and doing random acts of kindness. I'm not convinced that the kids with dozens of presents experience Christmas on anything deeper than a materialistic level.
To be honest, the ideology of Santa isn't something we'd want to embrace in our family anyway. Let's face it, Santa is unfair. Some children get a small stocking, others get dozens of expensive presents. Some children get what they asked for, some don't. Does that mean they aren't good enough? There are millions of children in the world who are homeless or starving. How do I explain to my children that Santa gives nothing at all where it is needed most?
I do try to teach my children not to spoil it for other kids who do believe, but they may well slip up some day. I think there are far more important things to be "very pissed off" about than a child making an innocent mistake. But personally I think that children should be free to talk about their own beliefs, it's good practice for when they come across others of different cultures and religions (incidentally, it seems to be fine to tell children that God is not real!).
And just for the record as several people have mentioned it, actually I do know several adults who are still upset that their parents lied to them. It may not be common, but it does happen.