DocZ,
This area is often ignored, but it should also have an expansion joint. Joints are particularly helpful if the deck is poured all around the back of the spa.
If the deck is only 3' or so, then the deck installer may be counting on the nearest expansions on either side of the spa to do the job.
Concrete generally shrinks slightly after its poured.

Thanks golfgeek.... I'm more worried about where the deck actually meets the raised spa wall.. Will this put more stress on the spa wall or deck, and/or impact its integrity?

DocZ,
If the deck is poured against the entire raised spa then yes, it should have an expansion joint.
Location of the spa in relation to the pool and the area of contact between the deck and spa may be factors that caused your concrete contractor to choose the method he did.