It's hard to say either way unless we find a primary source (someone privy to the company's strategies), but I think a refinement of your question might be—
Are any ingredients for brewing either unavailable or prohibitively expensive depending on season?
I doubt this is the case—if an ingredient were impractical to obtain in other seasons but one, then I'd imagine the price of that seasonal beer would be extraordinary, which isn't the case with Sam Adams' seasonal beers.
If by chance there do exist such ingredients, then the next question is—are any of those ingredients used in Sam Adams' seasonal beers, which itself is a difficult question, since Sam Adams' recipes are proprietary. From Sam Adams' site, though,
Samuel Adams® Octoberfest is a malt lover's dream, masterfully blending together five roasts of barley to create a delicious harmony of sweet flavors including caramel and toffee. The beer is kept from being overly sweet by the elegant bitterness imparted by the German Noble hops.
Unless one of those "five roasts of barley" (providing the above is even true and not a marketing statement itself) is some rarity in other seasons, which I highly doubt, again, I'd say mass produced seasonal beers are only "seasonal" for the sake of marketing and inducing a certain feel—
Samuel Adams® Octoberfest provides a wonderful transition from the lighter beers of summer to the winter's heartier brews.
I feel guilty posting this as an answer, since, it's not a definitive answer, but I don't feel the question can be answered by anyone but a primary source, so perhaps this question should be closed instead—but I don't know what the appropriate reason would be.