How does Android Market DRM work? I have a bunch of rooted tablets purchased for use on a locked-down WiFi network (for security reasons internet access is not allowed). Using a different network, I purchased an RDP client app from the market. After a while on the secured network the app won't start with the error "This application is not licensed. Please purchase it from the Android Market". 

Can someone explain how this works? Android Market apps connect on a periodic basis to check licensing? This seems ridiculously overbearing. Is there anyway I can verify authenticity only at install time so I can run on our secured network?
There are two types of protection offered by the Android Market.

The original copy protection is now deprecated and is apparently fairly easily circumvented.

Google now recommends that developers implement its licensing service, which requires a bit of effort on the part of the developer.  It requires Internet connectivity and checks whether the Google Account being used on the device has legitimately purchased the app.  How often the app goes online to check the licence is up to the developer.