I don't know all the codes but in this case logically it just makes sense. Imagine drywall screwed to studs and moisture and water and mold setting in. What will hapopen? Well, it will weaken and eventually fail. Now if you add tiles to the mix you are adding weight to the drywall and as the drywall weakens from water and moisture ect.. it's going to start to crack and eventually fall. In most cases you would be able to punch the tiles and the wall would crumble. I'm not sure how long this would take but in any case it wont be too long before you are replaing the drywall and tiles with cementboard and new tile.
Get them to do it right the first time.
They also might have used greenboard(drywall that has a green finish on one side) This is by no means waterproof. The manufacturer states that it is water and moisture resistant BIG DIFFERENCE!! I see a lot of people using this for showers and this could pose a major problem in the future.
I actually have one of my basement showers lined with greenbaord and then a plastic shower over that. I checked the greenboard and it seems to be ok but it is not in direct contact with water.
This was not my work and it is comming down very soon but it is an interesting real life test for me. I'm just wondering how teh humidity is affecting it??