Ideally it comes down to whether the buyer knows the risks involved. I'd be satisfied if big warning labels (hey we got them on everything else) were on the package stating you could be, and people have been, seriously injured or killed using the device. Perhaps some statements about what not to do and what to do when you do it anyway and things go bad. Thusly warned people could be free to choose the risk vs reward ratio they desire. Of course this won't happen because the government believes it's job (partly) is to protect people from themselves. This leads to a loop wherein people stop thinking for themselves and start believing that the gov't will stop anything bad from happening to them. We are defeating Darwin.
How bad might a crash be ? Let's think about the dynamics of the situation for a moment ... how fast will a rider be going when he (or she) hits the water ? Let's neglect any slowdown due to air resistance and have the boat going 30 mph and the tube ~30' high* when the rider and tube part ways**. If I remember my high school physics correctly that would have the rider doing about 30 mph towards the water plus 30 mph forward so the combined velocity is about 42-43 mph. Certainly enough to hurt you but no more so than water skiing and "tripping" over the wake after a strong cut. Moreover the rider would have ~1.4 seconds to prepare, where my skiing falls have happened real quick.
His impact angle is a lot worse than mine though but then again if he had balled up before impact I doubt any harm would result (image a 30 ft bellyflop
). This is where a little forethought would pay off. And that would apply to the manufacturers as well. I betcha a little testing would have shown these things have a tendency to spin out of control and I'll wager a better design that reduces this tendency could be (and perhaps has been) done. Now it's probably too late for this type of product. Just my 0.02 ....
*just to bookend it, 25 mph and 15' has you doing 32-33 mph when you splash
** this isn't the worst case scenario but it's illustrative