Well, supplements aren't subject to inspection/testing like food is. There is no verification.
As for the lack of mad cow from people taking supplements, that is certainly a positive piece of evidence. I'm weighing that against the lack of testing. We don't want to find out later that, as the years progressed, one offshore manufacturer (maybe a subcontractor or partner to a local company) decided to scrimp on their own oversight, allowing infected products to enter. Past track record *doesn't* prevent that, especially in the absence of monitoring mechanisms.