They probably had to pick a number and 10 sounded nice enough... Smaller and it's probably considered a "trolling motor" or something like that (not being a power boater, I wouldn't be an expert on that).

I guess a 10 HP engine could get a small johnboat on a plane?

I suspect most folks actually do help their kids gain the requisite safety attitude before boating alone, although the few outliers will be the ones causing headlines (and further nanny-state regulations)

And perhaps if there were less wrongful death lawsuits for what should be properly classified as Darwin awards, we may not need all of this government oversight?

As in the incident involving the two kids who went missing, do you blame the parents for letting them use the boat... even though they told the kids not to go offshore (or whatever)? I wouldn't unless there was evidence that these kids couldn't be trusted in the first place.