Last I checked, native Android on the Pi is not very well supported, apparently the drivers aren't designed for it. Virtualization is one way around that problem. As for hacking the Swagbucks miner to run native on Raspbian, it would involve a lot of low level hacking - they didn't want to make it easy for someone to embed the miner in some other app.
One problem with just using a $10 phone, besides availability, is that developers have surprisingly little interest in rooting them. They rather spend the same time hacking a $50-100 phone and end up with a much better device. Also, the Raspberry Pi is the most popular "standardized" ARM platform that makers are likely to have lying around.