Seafood Raised on Animal Feces Approved for Consumers
If you are a seafood lover and wish that you could eat more fish raised on pig feces, your dreams are coming true. Due to fierce competition in the Chinese tilapia industry, farmers are increasingly switching to feces instead of commercial feed. From the article: "At Chen Qiang’s tilapia farm in Yangjiang city in China’s Guangdong province, which borders Hong Kong, Chen feeds fish partly with feces from hundreds of pigs and geese. That practice is dangerous for American consumers, says Michael Doyle, director of the University of Georgia’s Center for Food Safety. 'The manure the Chinese use to feed fish is frequently contaminated with microbes like salmonella,' says Doyle, who has studied foodborne diseases in China."
Before there was such legislation you could still restrict yourself to only purchase from companies that voluntarily got certified and voluntarily informed ingredients. No one were making you buy food from producers who didn't list ingredients.
There are several flaws in your hypothetical no-legislation scenario: