I don't understand why anyone would use passwordless disk encryption. It just seems inherently vulnerable, especially with the threat model of physical compromise.
Entering a password on boot isn't even that much work
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From their perspective, "Secure Boot" has the word "Secure" right in the name. And they've looked up details about the TPM - Microsoft says the TPM avoids systems being tampered with, and addresses the threats of data theft or exposure from lost, stolen, or inappropriately decommissioned devices.
If you don't know the intricacies involved, that stuff all sounds great! So they put a line into the corporate IT policy that TPM use is mandatory.