Apps that have free trial
All the apps that I have purchased that have a free trial all you do is download the license key. No actual changes are made to the application it's self.
To be fair, if an application developed for a PC or Mac could do many malicious activities if the developers wanted it to (it does not take special permissions to launch a second process that connects back to a C&C server and downloads additional code and a lot of newer games do this for legitimate reasons). In this respect an app developed for a phone could do a lot with just a few simple permissions if that was what the developers wanted it to. There is a lot of trust involved with downloading and installing an app on any device even if people don't think about it that way.
Bill
