3) It’s much easier for Apple to monitor and control signed software. Malware authors have to sign up for a developer account. Apps have to be uploaded to Apple for a malware scan before distribution to be notarized. Apple can revoke Developer ID certificates and notarization tickets at any time.
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That’s why it’s beneficial to give out free code signing certificates to all Mac developers, with lenient terms. Currently, not all non-malware is signed, because some developers can’t or won’t sign up for the Apple Developer Program, either because of the cost or because of the highly restricted terms associated with the App Store, which is inseparable from the Developer ID program.