I can't find any evidence USDA did this, but I do recall reading that the Japanese health authorities had. But the old levels, much like FCC RF exposure limits, weren't based on any science showing harm, just on the levels they'd typically see. That doesn't mean the new levels aren't dangerous, but it also doesn't mean the old levels were safe. It's all about cost of compliance. We just need more research to know what levels are genuinely safe or dangerous. Though to some extent the Japanese are providing this by acting as a living experiment. If there isn't an increase of cancers from eating food at the higher levels, then hopefully the higher levels are fine.