Spotify is Removing Songs from My Playlists

Spotify has been growing for a few years now and it's finally reached the US market. I've been using Spotify for quite some time now and even created a Spotify Drinking Game some time ago. For a while I upgraded to the Premium offer for £9.99 so I could listen on my Android phone. I've since cut back on some costs and I'm back to the Open plan.

The other day I went on the hunt for some music to add to a new playlist. I stumbled upon some continus mix tracks that last from 60-80 minutes for a single track. I added them to my Continuous Mix Sets playlist and enjoyed them for a day or so. All is good...

I've since came back a few days later and all of a sudden the tracks I were listening to only days before are no longer available for listening. I wonder now if this is down to licencing issues or perhaps a further limitation Spotify is placing on users due to the lack of advertising one would be exposed to when listening to a track of such a length.

I've reached out to Daniel Ek (Co-Founder) and Andreas Ehn (CTO) of Spotify via a Google+ post asking them about this and looped in tech evangelist Robert Scoble to get his opinion. Wonder if they'll have a reply at all, doubtful (Robert quite possibly though). What do you think; licencing issue or Spotify further tightening the reigns on free use?