The regiment guest workers had to abide by were detailed in treaties between the GDR and whatever country send the workers in the first place.
1.) From the point of view of the country sending them the goal was first and foremost to get skilled workers for their own economy, although this changed a bit through time. Giving them the opportunity to mix with the local population and having families would have inevitably led them to stay in the GDR, as the GDR had the highest standard of living among communist countries.
2.) From the perspective of the GDR there were many reasons, some subtle other more than apparent. For one isolating guest workers was an easy way to control them. The Stasi was acutely aware of contacts between the local population and guest workers. The relationship was on the one hand economical in nature. Just to give an example: the Vietnamese experience in sewing jeans was highly sought after among East Germans. On the other hand they did track romantic relationships.
While the Stasi tolerated these contacts most of the time, the reason they kept track was the fear that culturally different groups of people could be a possible source of dissent. Some guest workers, for example Algerians, had informal contacts to countries in the West which was an obvious source of suspicion.
Another reason, more subtle than the previous one, was ideological in nature. The GDR viewed itself as a German realization of socialism. While official propaganda emphasized the internationalist aspect of socialist doctrine the official policy was one of ethnic and cultural homogeneity. Multiculturalism was suspect and seen as a sign of "decadent" capitalism as it flourished in West Germany. So anything that went beyond symbolic celebration of the cultural differences among socialist countries was frowned upon.
Sources among others and all German:
http://www.politische-bildung-brandenburg.de/ausstellungen/als-arbeitskraft-willkommen