I don't think it is alarming. Check out most lists that rank graduate schools around the world. The US is quite disproportionately represented among the top. We've got Harvard, Berkekey, Stanford, Caltech, MIT, CMU, Chicago, Princeton, and UCLA for instance. No one else comes close to having that many schools of that quality.

The UK has two to four, and nearly everyone else has none or one.

Given this, it is not at all surprising that a lot of foreign students come to the US for graduate school even if they do not intend to stay in the US after getting their degree.

According to this article[1], 46% of American PhDs in science and engineering were awarded to foreigners in 2007. For 2011, I've heard 52%, but I can't locate the source.

[1] http://online.wsj.com/article/SB1000142405274870490560457502...