How exactly are they going to know they're being flagged as bots? When all you're doing is discounting those bot clicks to the advertiser it becomes far more difficult to test your limits as a bot (you would need to be a paying advertiser, just to analyze the human to bot threshold). It's similar to hell-banning, in that the user's generally unaware they've been hell-banned.
Take a minute to think about the effort a bot account has to go to in order to even come close to gaming a system that tracks their behaviour.
Let's assume that in order to be deemed human you need at least a few characteristics like "account older than 1 day", "friends from other non-suspicious accounts", "a photo or at least comments on a friends photos", and a "certain amount of click activity".
Now let's take that criteria and make it difficult for a bot to even know if they're being flagged as a bot where the only indication is a reduced bill at the end of the day / week / month to the advertiser. So bot makers would have to pay to test and would at best receive delayed feedback on their success.
Isn't this starting to seem extremely difficult, and generally unscalable from a bot perspective? It doesn't have to be impossible, there just needs to be lower hanging fruit, of which there are plenty should FB implement a system like this.
And should this fairly simple solution still have a few holes that a very elite few bot coders are able to game, you've still got tons of data and time to use to tune your algos.
It just doesn't make sense that one of their advertiser's is being charged for 80% bot clicks, until you start to question Facebooks motivations.