I have bought a new song on Google Play (using Opinion Rewards). Google gives me the option to download the song but I want to know if it's legal or not.
It's hard to say, since you didn't really buy a song, you got some kind of license to listen to it. You need to look at the legal part. Maybe you have to always listen to the some through some Google program; maybe you outright bought a license to copy and play as you want. For example, I found one thing that said in the access type field "Streaming and by permanent download to your computer and/or device", so you could download that.