FReeper donnalee suggested that Foley was being very careful about these contacts' ages to avoid the "under-18" problem. If the pre-18 IM contacts were not released by ABC, it's possible that they aren't as juicy as the ones they did release. Maybe they were just "overly friendly". They didn't suit ABC's agenda, so they didn't bother with them.
This is what it appears might have happened.
It does appear that Foley was at least cognizant of when his "friends" became "legal." That may even have been behind him asking about one's birthday---maybe it was even sexual innuendo---"what do you want for your birthday [when you are legal and can have homosexual sex]?"
It's still very gross and wrong to recruit and groom underage youths for a sexual relationship once they hit their birthday. But ABC may very well have pimped the sensationalism factor by picking out the most salacious IM and tagging it as having occurred when the page was underage.
Or maybe ABC was just incompetent and didn't distinguish between the IMs based on date.