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This is a privacy issue. If there was a Data breach, everyone will know that you don't trust someone you used to know.
You're on their platform, on their servers, it is unsurprising that they log your account's behaviour. The people who you unfriended would already know on account of how you're already gone from their friends list. I'm not sure what you're angry about.
That means you've agreed to their egregious policies that basically allow them to collect too much info.
FB are notorious for privacy violations.