The reason for the price difference is that the DR has a 18% Tax on the base fare which is ONLY due when your point of origin is the DR. When you book a roundtrip fare from the US, your point of origin is the US and there is no 18% DR tax due. The Southwest fee listings can be a bit cryptic. They call this 18% tax "Transportation Tax (International)" in the fee breakdown. In the case above, $180 x 18% = $32.40. If you book a flight on Delta, they have more detail on this tax along with the two-letter fee code, "Dominican Republic - Transportation Tax (DO)". So, in short, this is a real tax and Southwest is correctly charging it only for DR point of origin flights.
I think you meant to post immediately before your post and not mine.
I never thought or inferred that it wasn't being correctly charged or questioned whether or not this is a real tax. I merely pointed out the difference.