ISBN is for books. Is it a standard procedure to assign an ISBN for a Ph.D. thesis?
I suspect it largely depends on whether the PhD is expected to be printed as a book. In some countries (like the Netherlands) it is. Many printing services offer ISBN registration as a freebee. This is likely how most dissertations end up with an ISBN (I know mine has one because of this).
Other than this, I doubt many graduates go through the trouble of registering manually for a PhD. I don't know of any place that explicitly requires it. So in short, it's not expected, but it's not that unusual for a thesis to have one.
What is an ISBN? It's not for books, it's for published, commercial books. So this question is effectively "are PhD dissertations usually published commercially?" The answer to that is, of course, no. That's not to say that there are no PhD dissertations with ISBNs, but they would be in the minority.