Hello martin shaff,
Thank you for posting to the Community. That is an excellent question. With the purchase of a Slingbox you are out right buying the product, there for the device is yours and you can do as you please with it. You do not have to pay a monthly service to access the Slingbox. With this we offer a free SlingPlayer plugin to access the Slingbox on any computer.
Unlike other things, such as Netflix, Hulu and the like, you have to pay for the service and thus you can download and access the app for free. Since we do not charge a monthly service fee for the use of the Slingbox, if the customer would like to access the Slingbox on a mobile device (Android phone and tablet, iPhone, iPad, Windows devices, Kindle Fire devices) you have to make a one time purchase of the SlingPlayer app to access it on the device in question.
Hope this helps,
Sling Media Moderation Team