Pandora

Hi All,
I was wondering if anyone else can explain why I hear Pandora through Sonos completely Ad free? And I haven't upgraded to Pandora one. I was wondering if this is this a free trial of some sort?

This certainly isn't a complaint but I'm just trying to understand why this happens. If I play Pandoras native app through my iPhone or even my macbook pro I'll hear ads, and I'm cool with that, however turn Pandora on via Sonos and I've yet to hear any ads!?
I really did think I was going to have more of a technical first question on these forums but given all the great info on here I've got by with simple searches :)

Thanks for all the great info and great support forums!