Why does BusyCal require iCloud credentials and not use local calendar access?

BusyCal syncs over CalDAV - by doing so it is able to sync meta-data required by the app, and offers tremendous flexibility and additional features that it would otherwise be unable to offer it was to sync via the native Calendar SDK

The native SDK is not meant to give developers the ability to perform complex tasks including managing attendees to meetings. This is why one would buy and use BusyCal to begin with as it is a complete replacement of Apple Calendar in both functionality and features offered.

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