Bridges are not the norm. In most cases the crossing for the causeway is usually shallower than this specific one. This one was six to eight feet deep in the middle--quite deep--and the velocity, as Paul mentioned, when they went across was too fast for the contractor. The contractor made the decision because it would be his safety responsibility. Normally it would be more along the lines of what Paul suggested, that you could use large rocks and let the water flow through it.
We have used bridges and/or conveyors in other places sometimes. That's usually a cost decision made by the contractors themselves. The bridge itself was for navigable waters purposes. Actually, there was still a lot of water going through the causeway when it was first installed.