While the question about business model is legitimate in this case, I would argue with the last statement in your post: Some people do it "for the fun of it".
Just one example:
https://play.google.com/store/apps/developer?id=SECUSO+Research+Group
As for Blue Mail, there is a phrase in the
privacy policy that concerns me as well (and has not been mentioned yet):
"Some of the information Blue Mail uses to communicate with your device – like its IP address – can be used to approximate the device’s location. This information may be used to administer, analyze and improve Blue Mail."
In view of the other information about what they do, my question is: how exactly are they collecting IP address(es) of my device and how are they using location information? Are they analyzing that for each message, or do they continuously poll my IP?
Or, maybe it is about the access to their website via browser?