It's an attempt to have "suspect" cards and suspect players come to them.
Instead of security monitoring machines and cards they let the player gamble the Freeplay. The lock was already there.
When the voucher does not cash out you naturally take it to the cashier. Once it shows flagged they will tell you to wait for a supervisor. Security will surround you.
Of course if it's on your name, you just show your ID and the card may be unlocked by the host. Maybe not.
There are sometimes legit reasons a voucher isn't accepted. The barcode may malfunction and you do have to go to a cashier.
It's a tricky situation