The problem is that you are using docbook.sch for DocBook 5.0, which is not an ISO Schematron schema (the namespace bound to the s:
prefix is not http://purl.oclc.org/dsdl/schematron
).
In docbook.sch for DocBook 5.0, simply change this:
xmlns:s="http://www.ascc.net/xml/schematron"
to this:
xmlns:s="http://purl.oclc.org/dsdl/schematron"
...and it will work.
In docbook.sch for DocBook 5.1 (which hasn't been offically released yet), the namespace has been changed.