The glamour journals do publish a fair share of junk. Sadly the university administrators and search committees prefer Science and Nature over PNAS.
PNAS has a nice system that helps reduce (though not eliminate) the junk: The editor making the go/no-go decision after referee reports is an actual working scientist, a member of the NAS (of which the journal is the Proceedings). That means that while they're probably not a specialist in the exact area of a given paper, their areas of expertise are close enough that they can make much more informed decisions than the professional editors at Nature and Science. The latter usually have PhDs in their areas of coverage, but aren't active researchers, so their domain knowledge is more second-hand.