As far as I know this has been mostly about boosting "diversity" quotas at German Universities in order to perform better in international rankings. Previously this had been done by rising the amount of students coming from low-income regions (e.g. ex-Russian territory, Asia), so it is certainly a welcome change to see US students making use of that system.
In regards to the ranking - one of the most important rankings for international Universities is the Times Higher Education World University Rankings. Methodology section:
http://www.timeshighereducation.co.uk/world-university-ranki...this factor is measured by the ratio of international to domestic students and is worth 2.5 per cent of the overall score.
German Universities have a big issue about ranking performances, case and point giving certain Universities a Government issued "elite" title.