Were the Sam Adams seasonal beers actually developed to accommodate seasonal availability of ingredients or are they just a marketing construct? I enjoy the Sam Adams seasonal varieties, as they usually put people in the holiday spirit(s), etc., and they evoke the feeling of "only for a limited time" like a Shamrock shake at McDonalds.  

In that vein, were the beers originally designated "seasonal" due to the availability of certain ingredients during different times of the year (Harvest Pumpkin Ale springs to mind) or is this notion of a seasonal beer chiefly a marketing construct?
A few notes on brewing and the seasons.  It isn't a question of availability today but there are other factors that likely have seasonal impact.

Traditionally (say, 1000 years ago), yes, brewing recipes would vary with the seasons.  Some ingredients like henbane seed or barley keep extremely well.  Others do better  fresh (herbs and such).  It is likely that gruit components varied with the seasons.  This is not an issue today.  Aside from some components of other beers (pumpkin ale, beers with seasonal fruit), most beers today do not involve ingredients that are problematic to preserve over a year.

There is a second issue though.  That is that beers traditionally have another factor which is starch content.  The starch contributes significantly to calorie count of the beer, and usually darker, richer beers have higher carbohydrate count.  It makes sense to consume more calories in cold weather than warm weather, and it makes sense that darker, heavier beers would be favored in winter while lighter beers would be favored in summer (although as one would note the heavier beers came out of traditions of using them as bread substitutes for fasting monks and so would have year-round applicability in monasteries).

I don't think it has to do with availability.  I think instead it has to do with seasonal expectations and reserving richer, heavier, higher starch beers for the winter (we have higher metabolism in winter, see http://www.mate.tue.nl/mate/pdfs/4319.pdf, and generally less availability of fresh foods before modern refrigeration and rapid transport).  I don't know when this became common though.  In general, when I look back a thousand years ago, I don't see anything in primary sources to suggest heavier malts in the winter.  So trying to date this would seem speculative.  There is a fairly obvious practical pattern though.

EDIT:  As I think about this, I would note that aside from lagers, it is very hard to brew in the winter in a colder climate.  There may also be a component where heavier, stronger beers keep better and therefore keep into the winter better.