Wow - surprised this is so rare for many of you in the US. I see this quite often - most often coming to/from Orlando. I'll also say that I have seen it on airlines that seem to cater more to families - Southwest, the late Song, JetBlue. From listening to conversations, I think a lot of people that don't fly often are excited about the travel, and also, happy nothing bad happened. Seriously.
I have also experienced clapping on on very turbulent flights on various airlines.