Account should be locked once reconciled, but credit card transactions can often be dated close to the statement date without appearing on that statement. So locking on the statement date means those transactions can't be ticked off on the next month's reconciliation.
The lock date is driven off the statement closing date and whilst we totally agree with your scenario we would like to keep the date locked down (pardon the pun) to the statement date during the reconciliation process.
However, the lock date can easily be changed (if you have the correct permissions) by going to Banking > Bank Accounts > Spanner Icon
You can now change the transaction lock off date for that account.