Pro Referee USA Training & Deveopment Manager Paul Rejer posts the following video for the purpose of showing that you can have a PK called without the player going to ground.
He makes it clear that this is his interpretation not that of Pro Referee USA but he does make some good points. One being that if referee's called these kinds of fouls more often then players might not feel the need to go to ground so easily. 'These kinds of fouls' being though's that fall into the 'attempts to . . .' . In the video you see the Dallas defender attempt to trip but the DC player stays on his feet. I think Mr. Rejers reasoning for the PK is that without the attempt to trip the DC player would have kept control of the ball, etc. Read his article it brings up some good points.