This winter I had a pool installed with a concrete deck. About a month later, four of the deck sections began to crack perpendicular to the control joints. The contractor said that cracking was "just one of those things" and that he would not be prepared to do anything unless the cracking became structural in nature. Am I being taken for a ride about the cracking? Is there any route I can take to verify that the contractor workmanship or materials are not at fault?
Your concerns about cracking are justified. The reinforcing could be too near the surface and be what is called reflective cracking. If your concrete is suffering from reflective cracking the cracking will widen due to corrosion of the reinforcing steel as it is exposed to the elements it will rust and become a bigger problem.
On the other hand, let us say the cracking is from shrinkage, or the loss of volume by the concrete and the crack appeared in the mid-point of the broadest area. In that case the contractor is speaking the truth. It does happen, but it is normal to expect that crack and normal practice would be to put a "joint" in the same area to make it look better in the long run.
If you want more help on this matter, you can contact your local American Concrete Institute chapter and ask a local concrete professional to volunteer to look at the work. Maybe they will have some helpful things to tell you.
In the meantime, you may have to start filling those cracks with a flexible caulk to prevent further water incursion. Finally, invite your contractor over one more time, see if he'll be more helpful.