They come for studing in universities. They often get into financial trouble since their governments regularly fail to provide them with scholarships on time. This makes the foreign students take up illegal activities rather often. But the real reason of their position is usual racial hatred combined with the fact they aren't numerous enough to resist. The last condition is absolutely necessary for human scum, who beat the Black students.
It's not like their economy is so overheated that they have to import workers.
There are many people from very poor countries mainly from the former USSR, say, from Tajikistan. They take up (mostly illegally) in construction, on vegetable distribution warehouses, or as street-cleaners, road builders.
Russia is a poor country comparing to the USA, but for Tajiks, Georgians, Armenians, even Ukrainians, etc., it's a place where they can earn money.
Also unstated is exactly how much protection foreigners can expect from the local milita (police). Just about none, and if you happen to be unlucky you want to try your luck with the Skinheads.:(