They care if you find out your own status is because then if you date yosef and you give in your DY numbers and are positive, then yosef will be able to know that he is positive for the same thing you are positive for. And DY does not want people to be able to use their program to figure that out.
I asked why you wanted to know if you have tay sachs gene to make a point. The only reason to want to know is if it matters in some way to you. Since it obviously does, it should not be surprising to you if there would be a stigma.
And genetic testing is not as simple as you think. It isn’t a matter of simply putting it in a machine and it spits out all the genes. Finding the tay sachs mutation involves looking specifically for that mutation.