Support.
I see no reason as to why the resting period should start at any moment other than when the stud is bred.
The stud isn't doing anything except sitting there waiting for the listing to be purchased, and it does unnecessarily punish the stud's owner if they either change their mind or need to alter the listing for any reason.
I agree with several reasons above, including that this could possibly deter the use of the FM studding system for the riskier options.