before going into technical, when things come to politics they can do what ever they want to do with iran regardless of the laws! we have to face this, most of them(americans) hate us, and consider us their number enemy, the rest work for the companies owned by jewish community which hate us.
even before thte open  source annuncement sun  has no legal reason to block iran from downloading  java, other than considering java  very high tech, since JAAS is under export control restriction, but you could download .Net, Oracle, Novel Mono, and different linux and BSD's, which each included some more high tech products falling into same export control laws. I think thats just political.

going back to technical issues,

not ALL of jdk is open sourced at this time now.
they are moving into open sourcing all portions of JDK and JVM but tht process is not finished.
they mosy get ride of commercial codes and some legal issues to finish the process.
take a look at this URL:
https://jdk.dev.java.net/
this colored tree shows which parts are open sourced and which parts not.
you may refer to this url to get better information:
www.sun.com/software/opensource/java/faq.jsp


you can download JDK from java.net but not from sun because JDK is dual licenced the one in sun can be considered the commercial one.

other refrences:
http://www.sun.com/2006-1113/feature/
http://www.sun.com/software/opensource/java/