Comment by TrainedMonkey

Your genetic data is probably most accurate / valuable PII there is. Assuming multiple relatives use the service and at least one of them leaks online identity the whole jig is up.

So the issue is not tying it to your online identity, but rather them keeping a resource which becomes more valuable as the time goes on. So why is that an issue... most obviously because of genetic predisposition. There is always a temptation to sell diseases you are predisposed to insurers (and maybe employers... ugh). After that you can imagine someone figuring that genetics affects any number of things (sugar / weight / addiction) and sell that to advertisers...