The DR has the right to control its borders, but not like that. There's no justification for what happened in 1937 (in the same way there's no justification for all the people Dessalines massacred east of the border.)
Even Joaquin Balaguer, who was one of the most influential men trusted by Trujillo, said that "the massacre was a barbarous act by orders of Trujillo, comparable only to Hitler's massacre of the Jews in Europe." He did said that though barbarous, it did solved the problem of illegal immigration from Haiti until the PRD rose to power in 1978 and basically became desinterested in guarding the border. Balaguer said that many key people in that administration were bothered by the 'taint' Trujillo's barbaric act thrusted upon the international image of the DR, that they wished to undo that by allowing Haitians to enter the DR basically with no controls what-so-ever. He also said that, though with good intentions, a person doesn't need to be a political expert to know that an uncontrolled increase of the Haitian population within the DR would put the survivability of Dominican sovereignty and even jeopardize the very existence of the country. As the DR population increasingly becomes overwhelmingly of partial Haitian ancestry, the legitimacy of the border will eventually brake down and when that happens, the DR will cease to exist absorbed into Haiti in body, mind, and politically.
The recent issues that arose during the regularization plan and how the DR was effectively cornered by the pro-Haitians is seen by many Dominicans as evidence in favor of what Balaguer said would eventually happen. That is only a small taste of what's in store for the country if the flow from the west continues with little or no controls.