I'm actually less shocked that they're bringing criminal charges than that it's illegal to post a video making fun of the mentally handicapped. It's clearly in bad taste, but I can't imagine living somewhere where even the poster of the video had committed a crime.
I'm pretty sure that it would be considered a hate crime in the UK. This is an arrestable offense which can carry a range of punishments depending on the severity (e.g. physical assault vs verbal assault).
