Earlier today Gabe Aul replied to a user on Twitter asking if new install keys will be made available for the post RTM builds of Windows 10, the answer was "No," and to be able to install the newer builds from scratch (or clean install), an Insider must have already upgraded to Windows 10 build 10240 and activated it on that device.
This is all part of the new "keyless" installs that Windows 10 offers, if you have a valid and activated copy of Windows 10 on your device, you will no longer need to enter the 25 character product key with a clean reinstall, and it will activate automatically once connected to the Internet.
Build 10525 is the first post RTM Windows 10 released to Insiders, and while some may have opted to upgrade their RTM build to it, others may have decided to dual boot to it on the same device, or clean install it altogether; this method poses no problems.
But for new Insiders opting to install build 10525 in a virtual machine (VM), unless they have an activated legit license in the VM, the build will not get activated; the same goes for installing it on another PC without a valid license. However, if you were an Insider before RTM and have upgraded older builds to 10525 - as this user did, from build 10074 in a VM, then the build will stay activated.
Insiders that can't activate post RTM builds will have to install a licensed version of Windows 7 or 8.1, upgrade to Windows 10, and then 10525 from within a VM, or dual boot their device that has an activated copy of Windows 10 in order to test Wave 2.
There's always the evaluation period though.
Thanks for the tip FaiKee!