Cloud music apps won't sync from device to computer? I have a free Spotify account and can very easily sync music files that are on my computer to my Android device via wifi. But I can't find a way to sync files that are on my Android device back to the computer.

I've looked for a way to do this using Google Music and can't find it. Same with Amazon MP3.

If I download an MP3 on my phone, I'd like to add it to my computer's library and be able to easily sync it and then play it with Spotify, Google Music, Amazon MP3, etc., regardless of where I'm playing it.

Is there some reason the various cloud services allow you to sync from computer to device but not from device to computer? 

Any suggestions on easy ways to accomplish this with some other software? Or am I missing something?
It isn't possible to DIRECTLY upload songs from your device to Google Music, but there's an alternative way I can suggest you. Advice: is kinda a long process for something so simple, but, hey, Google's fault.
You need Dropbox, Dropsync (free for one folder on the market https://play.google.com/store/apps/details?id=com.ttxapps.dropsync&amp;feature=search_result#?t=W251bGwsMSwxLDEsImNvbS50dHhhcHBzLmRyb3BzeW5jIl0. but we don't need the PRO key for this, even if it's a great program and I suggest you to use it :P) and, sadly, a PC with Google Music Manager and Dropbox installed. It kinda defeat its purpose this way, but it's the only automatic way to do this, as far as I know. Otherwise you'd have to manually transfer every song from your phone to your PC.
You simply start installing Dropbox and getting it to work. When it's all set up, you open Dropsync and select the folder with your song inside and a folder on Dropbox, which shall contain only your music files. Done? Now you fire up your PC and launch Google Music Manager. Select the folder you synced with Dropsync and check "Automatically upload new song". What happens now? Everytime you download a song in your device, in the folder you selected, Dropsync copies it on your PC, which triggers Google Music Manager to upload it to its servers. So yeah, Google Music should implement a way to do this, definitely, but here's this workaround ;)