Gratuity for Catering

Help! We are grossly overpaying for catering (the catering company was required by the venue) and the catering includes a service charge of 20% so when we originally book we thought it was gratuity for the staff. Now its a month before the wedding and I verify that the service charge includes gratuity and the answer is no. The caterer said "Gratuity is not required but you can provide it if you would like". Is it wrong to not tip the staff when we are paying about double for catering that we would someplace else and are already paying a service charge? This will be another $2400 in cost for us if we pay gratuity