I think it probably should have been declared abstract, but once it is done and released it is very hard to undo without causing a lot of pain - see Java Language Spec 13.4.1:
"If a class that was not abstract is changed to be declared abstract, then preexisting binaries that attempt to create new instances of that class will throw either an InstantiationError at link time, or (if a reflective method is used) an InstantiationException at run time; such a change is therefore not recommended for widely distributed classes."
