Why Do I Need a Better/Lighter Road Bicycle to Train For 'Fitness'? I'm about to purchase a bicycle in the very near future and have been shopping around at different bike shops to see what I like and what they offer.

My primary goal here is to commute to work.  It's a 6 mile / 9.5 km ride (each way).  It's all paved road, mostly bike lanes, for the duration of the trip.  And that's what I tell the sales people.

Twice now, I've been asked if I intended to use my bicycle for 'fitness'.  And when I say, 'Well, sure, maybe it'd be nice to do that too...' the salesman points me away from the bigger 'commuter' bicycles and towards the smaller road bicycles (one said a road 'cruiser' would be best, the other just suggested a 'road' bicycle).  

I didn't think anything of it until I was at home - but now it's bothering me.  How does having a lighter, more efficient bicycle, that promotes a more aerodynamic position ENCOURAGE fitness?  Isn't that the exact opposite?

Surely for any given course/distance the cyclist with the heavier, less efficient, bicycle would be working harder.  I'm not a cyclist, but I do know if you go to the gym and use a stationary bicycle the more resistance you have the harder the workout.  

What am I missing?  Would you not get a much better workout on a heavy bike with an extra 70 pounds and sitting in an upright position....than on a top-of-the-line road bicycle?

Or is it just a reason to get me into a higher price-range?
Rob, you are correct that a heavier bike will give you a greater fitness benefit over the same distance.

The only real counter-point I have is that the most effective bikes for fitness are the ones that get ridden. So, if some reason a lighter bike would more fun or appealing to you (while still be a "good enough" commuter), than a lighter bike could be a better choice for you. 

In my experience, extra weight is not noticed much for commuting, so I'd rather have the durability of CroMoly over aluminum or carbon, and the practical benefits of fenders, racks, lights and bags.

An anecdote: My wife rides a bike with electric assist. You might think electric assist would reduce the fitness benefit, but opposite is true for her. She takes more trips because she has electric assist (including carrying one or two children plus cargo), and thus ends up riding-- and pedaling-- more than if she had an unassisted bike.

Some will suggest that you should get a lighter bike for "social" riding. That depends on who you ride with and what their fitness levels and interests are. I'm personally a stronger rider, and prefer to ride with my wife, parents and older friends, as well my children. For me, having a bike that's a bit heavier improves the social parity and helps to me ride naturally closer to the same speed with the people I prefer to ride with. If you want a hang with "Group A" club rides, lighter would be way the go instead. 

Whatever gets you out riding the most is best bike for you.