Not that rare: Here's just some, mostly from 2016 and 2015:

http://avherald.com/h?search_term=turbulence+injures&opt=0&d...

Otherwise, totally agree: compared to total number of flights (there's maybe 1000 B737 and A320 in the air at any moment), rare (though potential consequences could be grave.)

And, as you say, it's the negligence of basic safety protocol that's worrying.