That's not totally true. They do support scheduled shipments for folks like Apple where the items are shipped but held to insure basically a global US delivery date for most packages in the batch. But this is pre-planned, normally at worldport etc.
So if you had some buffer, you'd be buffering into more constrained capacity rather than less. Buffering works better if you buffer into a quieter period. That's not this sitution. So yeah, backpressure needs to flow to shippers in terms of pricing etc.