Great stat, now explain why his shots don't result in goals near as much as the players that you listed above. 4 of them are defensemen and score a lot of more goals. So, Petro shoots it on net for a good average, but his shots aren't effective.
Aren't we kind of shifting the goalposts here? He wanted to debunk a myth that Pietrangelo doesn't hit the net. Now you're trying to turn it into a defense of why Pietrangelo doesn't score goals as much as some of the higher scoring defensemen in the game.
Putting him on the spot aside, it's a question that's worthy of discussion. I have two points to make: The first is that a point shot on net is the (standard) definition of an effective shot for a defenseman. A shot from the point is simply a low percentage shot. The real hope is that there is a rebound, tip, or effective screen that prevents the goalie from seeing the puck. The defenseman scoring is the unexpected outcome. In that sense, bagging on Pietrangelo because he's getting shots on net but they aren't going in is kind of a backwards criticism.
You might be saying to yourself right now that not all shots are point shots. Quite so, and that bleeds into my second point, for which I'll turn to the ever-trusty War-on-Ice.com for help.
Here's a few images comparing the shooting percentage hot-spots for Pietrangelo, Doughty, Karlsson, and Keith, both in absolute terms and relative to league average. I selected all ES minutes from 2012-13 through the present (the Hitchcock years), both regular season and playoffs, as the best representation of "normal" shot quality and usage.
You'll notice right away that Pietrangelo's shooting percentages are right there with these other three guys when you account for where the shot is taken. A major difference in their absolute (sum-total) shooting percentages is that most of Pietrangelo's shot volume comes from the low percentage point area, while these other three guys have the green light to roam quite a bit more. This is especially true low in the zone in that high percentage scoring area near the net.
We all know that Pietrangelo isn't afraid to leave his point and go low in the zone when he has the green light, so convince Hitchcock to loosen the reins if you want Pietrangelo to score more goals (or simply jump on the "Fire Hitchcock" bandwagon).
Here's the PP graphs for the same time frame:
Same basic story. These other guys aren't displaying significantly more effective shots than Pietrangelo (to explain goal discrepancy as evidenced by shooting percentage), but they are being used much more in the middle of the ice and low in the zone around the net. Again, you can chalk that up to Pietrangelo not being willing to go low in the zone, or to him being told to stay high in the zone by his coaches. Personally, I think one is a lot more likely than the other.
In this case, there's also a fairly significant disparity in PP TOI/G which affects overall shot quantity as Pietrangelo has basically been a 2nd pairing PP guy behind Shattenkirk for the duration of the covered time-frame. Here's the PP TOI/G averages for the individuals above for the time-frame in question per NHL.com: Karlsson 4:33, Doughty 3:26, Keith 3:03, Pietrangelo 2:35. Giving up 30 seconds to 2 minutes of PP time every game to these guys is going to significantly affect bottom line numbers.
Individual shot characteristics aside (shot velocity, etc.), it seems pretty clear that Pietrangelo's shots are, at the very least, not significantly less effective at generating goals per shot on goal attempt than these other three guys when you account for where the shots are coming from. You can blame him for where the shots are coming from, or you can blame the coaching, but I hope we can stop just blaming the shot.
There you have it. Question asked and answered.