In most cases, it is extremely justified to cancel the debt of developing nations. Canceling their debt allows the developing countries to devote their time, talents, and money to things that most need their attentions, like infrastructure and education. When other countries help them out, that is allowing them to be put on the right path to success.
If by justified you mean that it is morally acceptable for them to cancel the debt the question is unanswerable. There are far too many variables in something like this to say it is entirely right or wrong. Not to mention that if America defaulted on any debt no one could do anything about it since they have spent so much money on their military that the benefits do not outweigh the costs.