Some apps are being distributed with secret Bitcoin-mining software embedded in them. Coins found are sent back to the app owners, of course.
And to make it legal, it’s part of the end-user license agreement (EULA):
COMPUTER CALCULATIONS, SECURITY: as part of downloading a Mutual Public, your computer may do mathematical calculations for our affiliated networks to confirm transactions and increase security. Any rewards or fees collected by WBT or our affiliates are the sole property of WBT and our affiliates.
This is a great example of why EULAs are bad. The stunt that resulted in 7,500 people giving Gamestation.co.uk their immortal souls a few years ago was funny, but hijacking users’ computers for profit is actually bad.