Hi HN! I work for Facebook's Public Policy Team focusing on our Security efforts. Unfortunately, this was indeed a glitch in our spam prevention system.
We integrate with many different external blacklists (https://www.facebook.com/notes/facebook-security/link-shim-p...) including Norton and when there's a block on their service we provide a warning to our users. I have already reached out to Norton to notify them, however, the cause for the block is a question best suited for their team. We only use our external blacklist system to protect our users and would never use it to reduce traffic for competitive reasons.
As Hanlon said: "Never attribute to malice that which is adequately explained by stupidity."