it would not shock me if you are right about koi food in terms of the safeguards. Gulp! Hopefully none of us will have to experience that.
The other reason not to feed dog or cat food, is that the protein levels are usually not balanced for a koi either. The average dog food is around 19-26% protein on average. Koi food is 35%-42% depending on what you get. Obviously that is a significant difference. The protein needs for a dog and a fish are different. You can find innumberable reasons not to cross feed foods from one animal to another.
That's not to say people don't do it all the time, of course. I mean, we've seen posts here where people feed their koi bread, as an example. Truly, that is something that adds very little nutritional value for a fish, but I hear about people doing it all the time. To each his own!