It is a complex question and involves a lot of different angles you could approach it from.
Most of the time, training knives do not have restrictions on the items themselves absent any context. However, doing certain things with a training knife can easily fall under other existing criminal laws. For example, if you threaten a person with a training knife when that person doesn't know it's fake, that's assault. If you make them give you something, it's robbery (possibly even armed robbery depending on the state). There are cases where a person was found to have a fake gun in their pocket on the street and was charged with conspiracy to commit robbery based on their behavior and other evidence about them (prior convictions, having ski mask etc.)
Metal training knives also can still be dangerous even though they are dull. There are plenty of murders with butter knives; put enough force behind it and even a round-nosed blade will penetrate body tissue.
It is worth noting that toy guns and airsoft also are not actually required to be marked by any laws (Airsoft, air guns and paintball guns even has a federal law mandating that they cannot be required to be marked). Makers just do that to avoid lawsuits and/or as general social responsibility.
