angel_saza,
I'm not a producer of commercial turtle food, so I'm not in a position to answer your question on behalf of those companies that do make commercial food.
However, I can make some assumptions. If I was running an abattoir, at the end of the day, I'd have scraps from slaughtered animals to scrape up off the floor. Now, I could either pay for those scraps to be disposed of, or I could make some money by packaging them up and labelling them as Turtle Dinner.
Just because the label says 'Turtle Dinner' does not make it good for turtles. But, I guess that most turtle keepers would simply read the label and assume that it is suitable for turtles. If people keep buying it, then the companies will keep selling it.
It is important to remember that freshwater turtles feed underwater, therefore anything that lives in freshwater is potentially part of their food supply. Cows, horses, goats, sheep and whatever else is in the Turtle Dinners have only been in Australia for about 200 years. In terms of evolution, it has not been long enough for turtles to develop the ability process red meat. That, and I've never seen a turtle launch out of a river, grab a cow from a paddock and drag her back into the river!
I'm sure that your turtle does love squid, but that does not mean it is healthy for the turtle. If you offer a child chocolate for breakfast, they'll love to eat it. But that does not make it good for them. Likewise, if you offer a man beer for breakfast, he'll drink it. If you offer onion to your dog, he'll eat it...
You must remember that captive turtles only have access to the food offered to them. It is up to the owners of the turtles to offer them a proper diet. Saltwater fish, frozen Turtle Dinners or pellets are not what I call a 'natural' diet for Freshwater Turtle.
I do agree that soaking saltwater fish in freshwater will dilute the salt. But I also think feeding them freshwater fish is a better choice.
Regards,
Michael.