If a query with a JOIN has several CONTAINS() predicates, then there's no way for the SCORE() predicate to specify to which it refers.
For instance in:

SELECT * FROM Document doc JOIN Document comment ON comment.docid = doc.objectid WHERE CONTAINS(doc, "foo") AND CONTAINS(comment, "bar") AND SCORE() > 0.5

we don't know if the score applies to the "foo" match or the "bar" match.

We can't just allow a <qualifer> in the SCORE() predicate because it will still be ambiguous when self-JOINs (like in the example above) are used.