Ah, I see.

So if you want to let user A share info with user B (and as you say, that's likely an essential use case for 23andMe), then 23andMe either 1) cannot let user B mess up, 2) cannot let user A share with user B, or 3) cannot protect user A.

Of those options, 1) is impossible, though they could perhaps have done more to make it harder. 2) ruins a major use case. That leaves 3)...