The Spotify songs are 30 second previews only. Even on playlists. You can have YouTube songs. However the YouTube Background Music app does that much better.

Adam Boe responded on 5/24/2017

Hello there! We're also disappointed we can only play 30 second previews from Spotify - but that remains a limitation of Spotify for the C# platform. Please join us and let your voice be heard on their GitHub issue for full song playback support: https://github.com/spotify/web-api/issues/57. The YouTube Background Music is great for YouTube songs/playlists, but we wanted to solve the problem of being able to listen to your own carefully curated Spotify playlists. Whatever your choice, we hope you continue to support the Universal Windows Platform!