It's at least partially ARM's fault, as ARM doesn't publish documentation for their cores. SoC vendors are just as bad, if not worse, but ARM doesn't get off scott-free.
Eh? Here's the TRM for the Cortex-A57, to pick the first arm core I thought of: https://developer.arm.com/docs/ddi0488/c/preface
The architecture manuals are also downloadable.
ARM doesn't mandate how TrustZone is used, it just provides the technology. If SoC designers use it to force DRM into their products, it's not really ARM's fault.
Also, just to make it clear, it is not ARM's fault if ARM based SoCs lack proper documentation.