I believe they did this because people were committing fraud. They were getting people buying several hundred dollars worth of stuff, then withdrawing their money from the account before the red card transaction cleared. Then they were returning it the same or next day and getting cash.
This sounds like an updated version of check-kiting.
In earlier times, a kiter would go to bank A & open an acct with $100. They would then go to bank B & open an acct writing a check for $500 from the A acct. Next they'd go to bank C using a $1000 check written from bank B & open an acct.
Rinse & repeat until the amt was pretty high. The kiter would then go & close all the accts before the checks cleared (remember back then it took 10 working days for a check to clear) drawing a hefty amt out of the last acct. By the time the checks began to roll in, the accts were closed & the kiter long gone.
Because spot takes several days to post debits from a guest acct, the guest could rack up a hefty amt in debits & return the merchandise for cash before spot ever got the money.
And the guest could close the acct before spot ever saw a dime.