Its down to licensing, an iPad is a tablet (greater than 7") and an iPhone is a mobile device, that is just the way the cookie crumbles. The move to allow on demand playback on tablets was in response to falling PC sales and the fact tablets are rapidly replacing traditional laptops. A phone is still a phone sadly!
To be fair, everyone should be happy - you used to need premium to use Spotify on both of them!
@josephd60 No you don't ;-). If you ever have that issue, then something isn't right.
You need a premium subscription in order to sync your tracks offline. Not to sync them between different platforms.