that is why i like my fcs/probox combo. FCS fins have a really weak tabs design so when it hit my body parts, it usually break off. I did the same thing too, somehow the board flip over and my butt landed on top of the fin, good thing it broke right off. It did cut through my wetsuit and a little red scratch on my butt.
For your case, i think they reinforce the future box, i have board that is set on future box that is set on light weight eps core and it would sink the box in plus break the fin off on a hard impact. I would rather snap a fin than fix fin box damage.