The TPM controls the rate-limiting. A compromised OS shouldn't affect it. (Otherwise a live CD could compromise your disk encryption, as the encryption is usually done at the OS level, not in the BIOS/EFI).
Besides, doesn't secureboot block an unsigned live-CD? Otherwise, I don't see the point of secure boot past vendor lock-in.
The whole "only applies to RT devices" was a bit shifty looking from MS, though. Fortunately the broken Windows on ARM model died.
Edit: I'd also note that, e.g. console gamers love this stuff. It allows them to pretty much rely on others not being able to cheat very well.