The comment belowed is a response from the landlord:
They left the apartment very dirty. It is clearly stated in the terms and conditions: “YOU ARE ASKED TO LEAVE THE APARTMENT IN A REASONABLE CONDITION, RUBBISH TAKEN OUT ETC. OTHERWISE YOU MAY BE CHARGED AN EXTRA CLEANING FEE” which is what they have been charged. This has nothing to do with the compulsory cleaning fee.
When my manager Tony Wilson did the checkout, he told them to please clean and tidy all their mess, put furniture back etc. However, they declined and an extra cleaning fee was deducted from their deposit.