Cash inflows from overseas are also a major contributor. The exemption of real estate from money laundering regulations makes it a sink for a lot of shady revenues.
2. Even if it is, housing is actually an amazing American export: http://www.slate.com/blogs/moneybox/2013/05/19/exporting_hou..., though it's rarely recognized as such outside of the literature.
And given the lack of required information & intentional ignorance ("so, Mr. Boriskypov, would you describe yourself as an oligarch?") I'm not sure how that dataset could reliably identify "international" purchasers.
I think a more appropriate criticism would be to focus on the narrow class of purchases (cash only) or small sample size (n=300, though the company represents 10% of the real estate market in the SF Bay Area). Unfortunately, it's the only published information I could find.