When I received my nexus 7 the first thing I did was put some music on it from my itunes library that I had previously exported and started looking for more music that I heard on a long road trip I went on the day before and was sadly, disappointed the songs I am was looking for were not there. So immediately I searched for an alternative and amazon, the last thing I wanted on the anti-Kindle was my choice and was pleasantly surprised to see both songs were only 25 cents a piece! I was stunned and now amazon has gone up a level in my heart, but I still don't like them.
Warner Bros. music is not in the library for purchase yet. Havent had too many issues with it, but there is a lack of some artists, most likely the ones you heard. There is nothing stopping you from getting them somewhere else, and then uploading them to your play music account, it just stinks that there are extra steps necessary to enjoy Linkin Park.
I'm a big fan of both Google and Amazon (companies), so whenever I'm in the mood for music, I check them both and whomever has the best price wins. If they're priced the same, I'd probably go with Google Play, as it hosts a vast majority of my music. Both have great deals from time to time, but catalog-wise, Amazon has more choices. But for my music tastes, they both carry what I need, so price is generally the determining factor.
It's a licensing deal. Hopefully they will get it worked out someday. Only Apple has Beatles, though, if I'm not mistaken.
One thing I noticed with Google Music is that the files are max MP3 quality at 320K constant bit rate, whereas Amazons are ~256K variable bit rate. Not a huge (or even audible, likely) difference, but still.
That also means Google's files are bigger, and they have bigger album art making them even bigger still.