Secure boot's intentions are to ensure a malicious modified OS cannot run (when taken at face value, there is also the potential for vendor lock-in, but noone will defend it based on that).

Remember that I-phone the FBI wanted apple to hack? With this golden key, anyone could've hacked into that I-phone.

I am not sure how e.g. bitlocker uses the TPM, but it might be that being able to load a modified OS allows for bypassing OS-implemented rate-limiting on TPM access.