The reference implementation contains a range of abstract base classes for every vital element defined by the JSR 363 API.
While some JSRs offer abstract base classes in their public API, many of these classes here are specialized to their target environment (Java Embedded and ME Embedded in case of the RI) so having them as part of the implementation makes it easier to scale and optimize for other environments in a different implementation.
E.g. an alternate implementation (not the RI) for Java SE 8 and above could use Lambda expressions or new classes introduced by Java 8 while the RI won't do this unless they become available in Java ME Embedded, too.