Even for chromecast and spotify I never want or use these features, and if I am not turning such a feature on I never want to be asked about a permission related to it, as a user. If I am turning a feature on or I am trying to use chromecast, THAT is the time to prompt me about it
It's a user experience thing. They don't want to show the cast icon to the user if there is no device present to cast to.
(I think both POVs are valid, but the actual solution should be for a proper OS-level interface to provide mDNS discovery...)