Good question. I'm not 100% sure, but I believe it has to do with the fact that the primary purpose of a norm-referenced test is to rank and sort students along a bell-curve. These tests don't actually try to measure what the schools are teaching.
The standards-based tests individual states in the U.S. are now using, as required by the "No Child Left Untested" act (can you guess how I feel about that particular law?) are not norm-referenced, but they have a lot of other problems--one being that each state gets to construct and implement it's own test, and the tests are not comparable across states.
Here's a link that mentions briefly some of the problems with the PISA testing mentioned in the OP:
http://www.fairtest.org/examarts/Winter 2004-2005/International.html