Why can't I add live radio stream urls to a Playlist?

It can't be added to any speaker.  It is added to the system. It will work with any Sonos speaker.  Why do you think otherwise?

Because by their very nature, radio streams are not like a playlist.  Example: When is the “playlist” containing a radio URL supposed to switch to the next entry on the playlist? 

I have just checked and I cannot see the option to add a radio station any more. It was there a week ago.

This could get complicated! 

I'm using ZP Controller app on a Galaxy watch to control playback.  This has Playlists as a category.  It doesn't have a My Sonos category.  So there's no way to control a radio stream without first starting playback via a sonos phone app.

Wouldn't it be possible to have named Playlist files with just one url entry, eg. NDR EINS.? 

So Sonos should enable a function that makes no sense so the user of an obscure, 3rd party, unofficial watch app can work around the fact they don’t list My Sonos favorites?  How about you write the ZP folks and ask them why they don’t support favorites? 

If I create an m3u file containing the stream url and then access it as an imported playlist, maybe this would work... 

Found it. 

https://support.sonos.com/en-gb/article/add-an-internet-radio-station-to-sonos

The option seems to be greyed out in the desktop controller but is available in the mobile app. 

Whether this will appear in your unsupported third party app is another matter 

I have contacted the app author.  

Why be so argumentable and sneering about this? 

Because requesting things like this is silly entitlement at its worst.  Read the room, dude. 

Which room? And don't call me 'dude', dude. 

You really are intent on making a impression with fellow users with your helpful and supportive attitude. 

Thank you for your advice.