Eric - Seems to me that local codes may apply. The big critics are from Florida and they do require concret which helps hold the tiles down during winds. In Texas codes may not require what is required in Florida...You may have to check with a roofer you can rely upon or the local Building and Safety department.

However I do agree that the gap is excessive.

To my knowledge at least where I live in CA concrete is not required. See it alot in the true Spanish Tile roofs but not for the flat concrete tiles.