What is it then?
My belief is that it was a tactical (hattip: FReeper apillar) decision by the Chief Justice. Kennedy was already voting with the liberals and thus the whole AZ law was heading to be overturned, the Chief Justice was able to persuade Kennedy to uphold the 'check your papers' immigration check part of the law during a legitimate stop by law enforcement. Roberts joined Kennedy on the ruling to achieve this.
Additionally, if the Chief Justice also similarly partners with Kennedy on the 0bamacare ruling (it is reported that they are likely co-authoring the opinion) to throw it out entirely, this concession on the AZ ruling will seem brilliant.
I guess Roberts adopted two children from Ireland via Guatemala....so he is sensitive to the “immigration” issue.