Why are C# 3.0 object initializer constructor parentheses optional?

It seems that the C# 3.0 object initializer syntax allows one to exclude the open/close pair of parentheses in the constructor when there is a parameterless constructor existing. Example:

var x = new XTypeName { PropA = value, PropB = value };

As opposed to:

var x = new XTypeName() { PropA = value, PropB = value };

I'm curious why the constructor open/close parentheses pair is optional here after XTypeName?