Take heart. It does not happen in all the cases. By law they are allowed to charge for excessive holes which will require filling and repainting.
The holes you make for curtain rods can result in a couple of big holes or a few small ones. You can take those rods off yourself, fill in the holes with putty and touch them up with paint. If you don’t you mind end up getting your security deposit deducted more than it will cost to repair them.