Snippets don't work in auth sign_up and auth forgot_password email templates

[greyskin]

If a snippet is included in either the auth sign_up confirmation email template or auth password reset email template, triggering an email via either of the relevant forms results in the following error:

Liquid error (line 1): This liquid context does not allow includes.

Is this expected behaviour? We use snippets in our email templates for email sent via the Actions API without issue. Is this specific to the auth email templates?