IMO it's extremely presumptuous to thank someone for that. You likely haven't a clue to the complete situation (maybe they were abused as a child), so no, you shouldn't thank someone for not having a kid. That said, it should be perfectly fine with everyone should a woman or a man not want to have kids. Not everyone wants to be a parent, or wants all that comes with that. That is perfectly understandable.
I think the entirety of the thanking someone you don't know for a basic life choice is incredibly patronizing and presumptuous, and makes you look incredibly self-serving. I've never known a war veteran who has appreciated when you 'thank them for their service'. It's vapid and empty, and it only serves to show your own opinions, or every less. It's for the person saying it, not as an actual extension of appreciation.
I started to read it as the author "congratulating" me for realizing I could never hack it as a parent. Why is reproducing seen as a "given?" It's 2013, shouldn't we have moved slightly beyond biological imperatives at this point? I once read a study that said art criticism is more a reflection of the viewer than the artist. Perhaps congratulations fall into the same category.