It is quite true that "espresso" is a brewing method and not a roast. "Espresso Roast" is a marketing term and nothing more. If you want to get very technical about it, "espresso" isn't a product at all. When we order "an espresso," we really ought to be ordering "an espresso brewed coffee," but I'm willing to bet that we all would find that rather pedantic.
I don't know if Sweet Maria's means that those beans are suitable for a dark roast or just have good flavor for an espresso, although I don't know what the heck either of those really mean. If I had to bet, I'd go with "suitable for a dark roast."
Espressos tend to me made from darker roasted beans for a few reasons, including:
(1) Tradition/expense- Old school espresso has been made from dark roasts (or we like to think it has). A roaster also can use cheaper, lower quality beans for such a roast, since nuances (good or bad) are pretty much gone in a dark roast.
(2) Body/acidity- Darker roasts tend to have more body and less acidity, which is important for a proper espresso.
(3) Flavor/Sweetness- If your roast is sweet for another brewing process, it can be -too- sweet for an espresso, producing an espresso that is cloying and offputting. Darker roast flavors also tend to blend better with milk (for a latte or cappucino)
(4) Production- Lighter roasted beans retain more moisture and are more dense than darker roasted ones, so it takes a little more pressure to make an espresso from a lighter roasted bean. That's not a really big deal unless you are dealing with an automatic maker or a "one-trick barista" who only knows how to do one thing a certain way. Darker roasts also tend to be more forgiving, in a sense, because so many consumers still expect their espresso to be bitter and burnt.
That said, the trend for lighter roasts has made its way to espresso, thankfully, and folks are making espresso out of all sorts of stuff. My current favorite espressos come from Full City roasts, which is actually what many Italian espressos are made from. I have no qualms in lighter roasts that aren't too acidic. I'm also fine going as dark as a Vienna, but any darker, and I may as well drink it from a dirty ashtray.
@Atalanta , I would have loved to try that light espresso from your old shop.