It is fairly easy to create a trivial ActivityPub application that consumes everything published to it. These are indistinguishable from intermittent personal instances that people spin up to test or read without establishing an identity elsewhere.

Sites tend to only defederate if objectionable content is pushed to them and their users complain.

I would assume that data scrapers are already in existence and have been running in some capacity since the first activity pub servers started gaining attention.

Attempting to block low activity or read only nodes would impact people spinning up local instances to use and try and lead to whitelist only federation.