That doesn't make sense, if this would be true how come they are still using metal forks?
The incremental costs of adding the knife to be washed with the fork and spoon is relatively minor vs. scrapping using metal utensils at all. While i haven't run the numbers, i find it hard to believe that there is economic value in substituting the knife only and not all the utensils.

Also, it doesn't explain why the european airlines (Scandinavian, Air France etc) all use all-metal utensils on all flights except the US bound flights where they use plastic knifes but metal forks and spoons.

This doesn't add up.