Naismith fell on top of him and was trying to get back up, he wasn't holding him down.
And i know what the rules say, but it's only recently that refs have changed the way they give free-kicks. They used to always give a free-kick or a pen for accidental handball, and a booking as well if it was deliberate. You can say it's unfair to punish someone for an accident, but not all fouls are deliberate either.