Is there justification for positive writing in a message refusing credit?
I'm not sure if I understand your question correctly, but if you mean should positive wording ever be used when informing an applicant that their credit has been refused, then yes, absolutely there is justification for it.
Consider the following:
1) Receiving word that your credit is not worthy of a loan or credit card is a negative experience. No one enjoys hearing that, as it is insulting on some level even if a person does have poor credit. That doesn't mean their credit will be poor forever, and when it improves, as a business you would certainly prefer to have them as a customer or client later on, so adding positive language such as "We look forward to your re-application in the future" costs you nothing as a business owner, and leaves the bridge with the potential client intact.
2) Most would consider positive wording in some fashion to be more professional. This is business, not personal, so to send correspondence that is purely negative serves to insult and humiliate. In business, what would be the point of that?
A positive attitude and courtesy are always desirable, and particularly so in business. Even when a company is unwilling to extend credit to a prospective customer who asks for it, there is no justification in assuming that the sale to the customer cannot take place.
A company will do well to act positively on the assumption that the customer will still buy in spite of being refused credit. Even when a customer decides not to buy the company should leave the door open for the customer to come back for other requirements some time later. A positive and courteous approach towards customer in declining any unacceptable requests from them helps to maintain a positive long term relationship with customers.