Definatly not something a decent builder would do, but we now live in a scoiety that can’t wait for excellence it has to be done yesterday so this is the kind of workmanship we all must deal with. Was the drywall used even greenboard? When you tear it all out check to see if they even used a shower pan between the subfloor and thin set morter. Check your blending valves quality check the quality of the shower drain. You will probably find that using drywall behind tile was not the only thing that was scrimped on. I think you should contact the contractor licensing department and after you find out who did this work file a complaint against whoever did the job that is providing they even had a license. I feel your pain