You can't prove something is safe, because there might always be future knowledge that you don't yet know. A sticker that repels tigers will seem safe until the day you are actually attacked by a tiger. But only one attack is needed to disprove the premise that stickers keep tigers away.

In this specific case, the antibiotic agents started being added to soap decades ago. Of course we know a lot more about biology and health now than we did then. The FDA last year asked companies to prove they are safe for long-term use by the standards of 2015, and that is what they failed at. But obviously those standards were not available to guide a testing regime in the 1970s.