There isn't a lot of reason why they would make an exception in this case. Apple doesn't want you to plug the Chromecast dongle into your Apple TV - it would prefer you to use its own streaming services. Google doesn't stand to gain anything by you plugging the Chromecast dongle into the Apple TV.
Moreover, neither do you, and this is key. If you've got a Chromecast dongle, you can plug it directly into your TV: you don't need to use the Apple TV as an intermediary.
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