From the article:

  The closest I got to an on-the-record comment confirming that
  press access was based on favor, not fire code, was when a YC
  registration official rejected my request for a badge, for
  the final time: “It’s an invite-only event for investors, and
  only a few select press are invited.”

The (paywalled) article states that they attempted to get press passes but were denied, so they snuck in. Perhaps the size of the event meant that YC wanted to limit the number of press? I don't know if they have a reputation for being disruptive, I just found the whole article petty.