Not paying your debts is immoral. Sure the risk is priced in but so is the risk of shoplifting in the prices you pay at stores. That doesn't mean that because the risk is priced in that it's somehow moral to shoplift.

Also the reason for default does have some bearing on morality -- a person that declares bankruptcy because of years of buying a new TV every year and going on expensive vacations is much different than a default due to factors outside of one's control (such as being laid off or suffering a medical catastrophe.)

Puerto Rico has, for years, knowingly overspent without much of a fiscal plan to pay debts. This issue isn't due to forces outside their control, but specifically because of decisions they consciously made. It is political malpractice and the people of Puerto Rico will now pay the price for generations. If that isn't immoral, then immoral has lost all meaning.