I don't know if this is still true(but this was certainly true in the past), but here is the story.
Due to Java License restrictions earlier, the JDK cannot be provided alone by vendors like IBM.
I am unable to pull this info from any IBM site at the moment but i am pretty certain about these license restrictions in the past.
Here is a post by another gentleman in this space:
From : http://www.ibm.com/developerworks/forums/thread.jspa?messageID=14514070
Unfortunately you can get hold of the JDK only as part of another IBM product (say, Websphere or any Rational product) that you purchased. Our licensing agreement with Sun/Oracle forbids us from providing direct downloads of the IBM JDK on any platforms that Oracle/Sun also support (namely Windows and Linux). If you look at the Java downloads section of the developerWorks website, you'll only find SDKs for AIX, z/OS and Linux on System p/z, since those are IBM owned platforms that Oracle doesn't support.
Since the JDK is shipped along with Websphere/Rational/Tivoli products, you can use it if you already have one of them deployed(though even then you may have SR8 FP1, unless you also install the very latest fix packs for those products).