The KJV translators did not do this also, (at least uniformly in all KJVs) so in itself lack of capitalization in the NIV is not a good argument for KJV only beliefs. Thought I would say that before the KJV Only people replied. In fact, I own 3 KJVs. He/Him/His in reference to God is only capitalized in one out of the three.
Capitalization is a rule in many translations, however: I have seen several if not most editions of the NKJV and NASB that follow the idea of capitalizing God's Name.
I believe that I also did once see an NIV where He/Him/His were capitalized. I don't remember the edition though. However, in the NIV, the actual Name (LORD or God) is always capitalized, as are nouns of direct address to Him.