Well I think you could provide both - a clear ending of the narative does not neccesarily imply that you can't allow the player to close up misc. loose ends beyond that event. Many platformers (athough not known for their great plots are at least comparable in quality to Puzzle Quest) have a clear ending, and then allow you to return to the game and finish off all the option quests and 'collect-a-thons.' Racing games can have a Final Cup that shows you victorious above all other racers and then still allow you to re-enter old races. Hell, they could have just unlocked some sort of "Free Mode" if whatever they wanted to provide plotwise did did make further gameplay implausible.

I re-killed giant living stone colossii by standing over their dead bodies and recalling their memory -- complete with an 'Olde Timey' post-effect!!!

There's plenty of viable choices -- I think that it simply did not occur to them that it was important. I think they figured that just by providing any narative at all that they were giving players a lot (which is true, in a way) but the danger is that when you do that you really should also provide closure...

Or.... they simply might have banked on the fact that only something like one-fourth of players ever get to the end of video games anyway....

By the way, Soup, your Freecell example made my instantly think of the old Breakout cabinet art:

I used to imagine I was busting out of prison.