Why it hits millwork hard
A shop can be deep into a job before the building is ready for install. Custom materials may have no resale value. Shop drawings, programming, finishing, and procurement are not free just because the piece is not yet installed.
A harsh convenience termination clause lets the GC cancel and then argue that only visible jobsite work counts.
A better payment rule
If the GC wants flexibility to terminate, the shop needs protection for real costs already incurred.
Ask for payment covering performed work, committed material, reasonable overhead and profit, demobilization, and closeout costs. That keeps the cancellation risk from landing entirely on the small business.