The murder RATE is calculated per-capita.
They do it by city, because within a given country the PER-CAPITA murder rate varies a lot.
Obviously in many small towns in the middle-of-nowhere in the USA (and other countries) the murder rate is tiny, which may drag down the rate for the entire country. But that is irrelevant to people living in certain big cities. Country-wide rates are irrelevant to most people.
Anyhow, the fact is that not a single Dominican city is among the top-50 by murder rate. By comparison, Honduras -- smaller than the DR by population -- has #1 and #5, while Jamaica -- even smaller by pop -- has #26, and Guatemala (only about 50% larger than the DR by pop) has #25. Obviously, population or physical size is not the issue. Besides, several countries have many cities in the list each, including Brazil with 19 in the top 50!. The US, from where many expats come into the DR, has 4 in the top 40.
Most of those cities are not places where many expats choose to live. Most of these murders are also gangrelated.
Would be more interesting to know how many expats/retires getting killed in the world and where? (DR would be higher than Honduras (besides Roatan) where you wont find many expats).