Hi avatar_kiyoshi,
Thank you for your interest in ATOMBIT Wallet! We delete private keys because we have devised a way using math to recreate them from benign data. This data is geographically separated, with the last piece being stored in the user's memory. What allows us to perform this magic is Zeropasswords, a security company that has gained wide praise for their innovative work in private key management.
If you have any further questions or comments, we'll be happy to answer them!
The ATOMBIT team