should concrete block wall be sealed on top?
Just a guess, but I'd say it was purely a cost saving measure. The sheet metal cap is sloppy, IMHO, but their might be anesthetic reasons for using metal. Personally, I would have gotten concrete wall caps and sealed up the top using those caps. I'm not sure I would fill up the inside if the wall was not load bearing or a retaining wall. Before I filled them, I'd inspect the wall thoroughly for cracks. 

If the wall is not load bearing or retaining, one potential risk when filling them up with concrete is that you will more than double the weight of the wall which might trigger settling. I have never worked with foam fillers, foam would certainly weigh less but might be harder to inject.