The "I" fares were put in place initially to match Virgin America, IIRC, and are the cheapest fares typically offered on this route. They often have long advance-purchase requirements and are nonrefundable.

The KUPs are not usually valid on the 3-class nonstops. They'd be good for the 2-class nonstops on EWR-LAX, or any connecting flight. You'd need one of the (much) more expensive "Y3CUPxxx"s or similar to get into J on a 3-class nonstop JFK-LAX. Sometimes these book into "I" inventory (and are non-upgradeable) and sometimes they book into "D" inventory, you can't really tell from the fare basis code.