IT IS NOT OPEN SOURCE - though you do get read access to the source code. You're right that you aren't forced to use their services, but you are also NOT permitted to use other cloud providers. So, it's either AWS or your own hardware. Still, this does seem rather amazing. One thing you left out is that there are no revenue sharing requirements either. You're not even required to use this for game development - you can use it to build movies, training simulations etc.