Are there good reasons for a public constructor of an abstract class

The answer is the same for java:

THere's no reason for a public constructor for an abstract class. I'd assume that the reason that the compiler doesn't complain is as simple that they just didn't spend time covering that since it really doesn't matter if it's public or protected. (source)

You can't call a constructor of an abstract class from anything other than a direct subclass.

So adding a special rule for access modifiers of constructors of abstract classes wouldn't add something useful to the language.

---

One thing that looks like an exception from this rule - if the abstract class only defines a default constructor, then the subclass does not have to implement a constructor: this is legal:

public abstract class A {
  public A() {}
}

public class B extends A {}

So we can create a B by calling new B() - but note, that we still create a B and not an A. And, again, it doesn't matter if the constructor in A is public or protected. It just shouldn't be private, but the compiler will notice and complain...

Actually we invoke an "invisible" public default constructor on B which does a simple super() call...