Google Play Music is a lot of different things.
You're talking about the "Radio" feature, which is, amazingly, free.
My estimation is that they cannot give you all one artist free because then you'd never have any reason to buy the music of that artist. The artist would object and it's not in Google's interest.
They have other services. They sell the albums of those artists, digitally. If you could get non-stop streaming of that artist, you'd never need to buy the albums.
They also have the All Access subscription service for on-demand streaming music, for a monthly fee. with that you could probably stream all one artist because you're paying.
The "Radio" stations are mixes of similar artists. It's pretty nice that they're free.
If you have Amazon Prime, you get an amazing deal on free music with the Amazon Music app. You can stream millions of songs, countless complete albums and add them to your online library... you can even download them to your devices, all included the annual cost of Prime. I assume if you cancel prime that you lose the library and downloaded music on your devices.
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