Solved! Go to Solution.
When you are a "free subscription user", you don't have the same privileges as a premium user.
If you are using Spotify on a mobile device, it is only possible to:
However, if you use Spotify Free on a computer, then it is possible to play ANY song, on-demand - without creating any strange playlists.
But if you want to play any song on-demand on a mobile device, then you would need a premium subscription.
To conclude; what you are talking about is not a problem/bug/error on Spotify, but just a prevention against exploits, and a marketing trick to compel free users to buy a premium subscription.