Staff asking for tips in Restrooms

I was asked for tips in the bathrooms in OR Tambo by the cleaning vstaff. I know they work for a contrator not for Airport SA . I was shoking and this happened on 2 different rest rooms near the boarding gates for international flights. In 1 there is a towel on the hand washing station with few USDollars from other passanger with few coins. I was shocked and puzzeled.am not sure if this is allowed .