Why should I pay for features that I can get for free with other alternatives? Probably so I can now pay Google to have everything in one place so they can mine more usage data out of me...
Background audio - jailbreak (free)
Google Play Music access - free if you upload your own tracks, if you want free streaming use Spotify
No ads - jailbreak (free)
Saving offline - same as above
I'm sure there are other tweaks on Android that allow the same features for free.
You currently don't have to pay a cent to get all the features (okay, sans streaming, but you can technically get it elsewhere) this subscription service is offering, and everything I listed is legal.