Why does the enhancer create a PROTECTE no-args constructor by default?

[KARR, DAVID (ATTCINW)]

I'm reading the OpenJPA doc, and I see that the enhancer creates a
protected no-args constructor if there is no public one.  Why exactly
does it do that?  What is the point of creating a protected constructor
instead of letting the JVM create the public one?  The doc only says
that it does this, not why.