Java (following Objective-C) does need a differentiation between interface and pure abstract class - this is because it is single inheritance - a class can have any one class to inherit from but it can have many interfaces.
That doesn’t follow. The restriction could have been defined in terms of allowing at most one parent class to be non-pure-abstract. And there are lesser restrictions hat would have been conceivable as well. Java is single-inheritance only in the sense of the particular interface–class distinction it makes. For example, since Java 8 you can inherit method implementations from multiple interfaces.
