They don't want to create user friction in their direction.

If facebook stops something working because of the ad-blocker, or accidentally cause something to break while trying to do so perhaps even for users without the blocker, the users will see facebook making it difficult for them to do what they want.

If on the other hand facebook stops the ad-blocker from being able to work on their site, without breaking other bits of the facebook experience at all, the same users will see the ad blocker failing to do its job.

Because if facebook blocked adblock users, then they would lose users that create free content for them, slowly but surely people would begin to abandon facebook.