i'll admit you got me curious... so i checked google scholar myself and found an article by Adam Kuper on jstor.org I've only read the preview page, but as an educated guess i'd say it's based off old customs, mainly to strengthen ties within tribes and form powerful alliances.

Anthropologists have consistently cited marriage strategy as one reason for the success of the dominant Dlamini clan. Thus Hilda Kuper (1949: 17) pointed to Dlamini-Tembe alliances in the inital phase of state-formation, and continued

Diplomatic marriages continued to be made for national purposes. Wives were selected for Mswati from many subject groups, and marriage ties were created, not only with powerful chiefs but with insignificant commoners. (...)

These marriages yielded, in the first place, useful alliances. "Female relatives of the king are politico-economic assests to be judiciously invested. The more important princesses are given to as wives ti foreign rulers and non-Dlamini chiefs. A wise king recognises the value of bunini (kinship). It extends throughout the nation, linking him by blood an marriage with chiefs and commoner, wealthy and poor" (H.Kuper 1949:58, 60. Cf. H. Kuper 1978: 344-5)

taken from Rank and Preferential Marriage in Southern Africa: The Swazi by Adam Kuper

http://www.jstor.org/stable/2801249?seq=1#page_scan_tab_contents

Thats all for the rest you'd have to use your own account to log into jstor or purchase the article

Keep in mind the studies are old. A lot could have changed but chances are, that a lot of the current regulations derive from old customs and traditions. As it is the same in western cultures.