Why are comments associated to a revision also deleted from tFlow when the revision is removed?

Within tFlow, comments and revisions can be added at the order level and/or at the revision level. 
When a job is added to an order, tFlow will always create the first revision even if the job does not yet include artwork.  This is done to enable customers the abilty to add commens to the revision and visually mark up the proof with circles, squares, pins, post-ts and pin drops.  These same mark up tools are not available at the order level since there is nothing yet there to visually highlight.
For this reason, when a revision is deleted - tFlow will also automatically remove all of the related annotations for that revision since they would no longer apply.