It was never a problem. Locking a memory card was meant to restrict the miss use of the memory card when not with the owner of it.
When you set a password on the memory card. The phone you use to set password, keeps it saved in itself to help you not give the password again and again. You will only be prompted to enter password when you use this memory card in some other phone. And as well in other phone when you give the password once it saves in it as well and then you will not be asked to enter the password in that phone too.
The trick told above actually removes the password (which was stored on the phone) while removing and restoring all the factory settings on the phone. That makes the phone forget the memory card and treat it as unkown. That's how the phone asks for the password to enter.
And the trick is only good enough till this stage. Now after giving the password, it again gets stored on the phone and will not be asked next time.. I am not sure how would he manage this again to use this trick repeatedly...