Customer Database. Stop customer databases being sold onto companies we never intended.

Most times when we fill out our details to some company there is the box saying click here to receive information from us and/or partners.
Then how does our information ie telephone and adress end up with companies not associated in any form
My wifes daughter was tragically killed five years ago, yet this week a brochure from next landed on our hallway. Now her daughter was only in this country for one year and that was years ago. Next apologised and passed on that they had gotten the details In December 2015 from a credit company. So the correlation between the two companies is? This also was upsetting for my wife to see her deceased daughters name appear especially as the only post received since her daughters death has been periodic bank statements.
Also in the days of mass scams by criminal else elements how easy is it for them to get databases that are just being sold on.
Our details should by law be kept by the company we gave the details to.