Cuban espresso is often made with robusto beans which are very strong. So your theory might be true.

Many cuban restaurants I have been to in Miami use Café Bustelo which to be honest probably wouldn't be so crappy if the roast-to-serve-delta was lower. Robusto beans get a bad wrap.

Café Bustelo at least comes in vacuum packed bricks, so roast-to-serve delta probably isn't that bad compared to most commercial coffee. In any case, it's not responsible for making me feel like I've just smoked 6 packs of cigarettes every time I drink one.

I'm so used to Café Bustelo from a moka pot that I find almost all coffee-shop espresso to be unpleasantly light. I actually pack the grounds extra tight and use lower heat to make sure I get an even darker and richer brew.

I hear you. I use to make Bustelo when I would occasionally smoke cigars on a nice summer day... even then I would add cream.

I'm almost certain that the primary reason is a general preference for sweets among Cubans. But it's possible that the dark roasts commonly used are part of it.