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.
If you're thinking of someone like MediaTek, Allwinner or the like: they aren't really in the merchant chip business. They pretty much design a chip for a single large project and supply it with support chips, (usually crummy) drivers, and even a linux or android port as a "complete unit". Often there isn't really a data sheet as such; they send some engineers along to help with integration.
This, by the way, was the original meaning of "SoC" when people were starting to think of common hardware/software codesign in the 1990s: a purpose-built chip for a specific design built out of a combination of standard IP and custom circuitry, but conventionally fabbed instead of being FPGA. Think of it as integration of some of the board design onto a single die. The IP market people thought at the time would eventually develop didn't really, though.
Intel and AMD are still in the good‡ old days of data sheets, second sourcing (pretty much dead but it's still in those companies' DNA) etc.
‡ and they were good times IMHO.
I think ARM SoCs are even worse. Try finding a full (or just partial) datasheet/technical documentation for anything like the ones used in recent smartphones and tablets, for example. They are going to want a NDA. The only ones publicly available were leaked. They also differ significantly between each other and often even within the same model line, so it's nothing like a PC where a lot of things still remain relatively standard.
ARM also has TrustZone, their DRM technology of which there is almost no public documentation.