I'm a brewer who's kicked around the idea of a micro-brewery/brew-pub in the DR . I drink Presidente when in DR because there's really few other options.
I do not pretend to know the specific brewing process for Presidente, but I suspect sugar is added to increase the net alcohol content. You can only convert so much starch to sugar to alcohol with basic ingredients. Adding sugar can give it an extra kick with just a little more yeast.
IMO, Presidente is just semi-fair beer. If you had a Presidente and a Rolling Rock side-by-side in a double blind test, you couldn't tell the difference. And Rolling Rock is not an especially highly regarded beer in the States.
FWIW-my favorite commercially available beers in the States (in FL, anyway) are Red Hook ESB and Sierra Nevada Wheat. I like Asahi and Sopporo for a dry pilsner.