Well I was going to say you should of bought a Ribcraft but it seems you have!!
I've got a 10 year old ribcraft with the same backrest and I haven't had any problem as I really give the boat a good beating
It may be worth sending the photo to Ribcraft and seeing whether they can comment to whether there was a batch that suffered from this failure. It may of been an inferior batch of tube.
If you bend it straight, it will probably happen again. Try Ribcraft as I have always found then very helpful.