Any cost conscious product manufacturer uses another NRTL for the small USA market.
Um... no. The US represents the single largest single market in the world, and is roughly 25% of the entire world. No manufacturer can afford to ignore a quarter of the market, any more than they can ignore the E.U. or China.
There are plenty of PSUs which have good safety, and are not UL listed (but UL certified by another NRTL).
That makes NO sense: UL Certification means Underwriters Laboratory did the testing. Another NRTL cannot by definition, UL certify anything
But assuming you meant that the product is certified by a different NRTL: You're ignoring the scope and purpose of an NRTL.
An NRTL can do testing for OSHA compliance. OSHA is only an authority for workplace safety, and nothing else. An NRTL's certification is only valid for an industrial or commercial application, and has no value for products intended for a home.
There are only 17 NRTL's, but even then, they are limited in scope. Each NRTL is only licensed to test a specific set of criteria: For example, the NSF is an NRTL, but it's wholly inappropriate for the group to certify an electrical product. There only a couple of NRTL's licensed to test electrical products.
It's also important to note the origin and continued primary business of UL: UL was formed by and works primarily for the American fire/homeowner's insurance industry. They are the laboratory that the insurance industry goes to in order to underwrite the safety of a product.
UL listing of consumer products isn't, and should never be mistaken for any sort of governmental certification. It's an insurance industry approval, and means you're likely to get a payout should the product cause damage.