Why? Because that's the cost of doing business.
The cost of replacements is borne by the seller. That's how business works.
I had a sit/stand desk delivered to my home a few months ago. The motorized legs started malfunctioning. The company came out to my home to replace the motor in the legs. They didn't make the desk, they were just the seller of office furniture. They had to obtain a replacement motor from the desk manufacturer. They had to bear the costs of obtaining that replacement part and sending out their service technician. That's how it works. I was not expected to drive the desk to their warehouse and have them install the new motor.
Apart from that, with the price gouging, particularly on shipping costs, that Sideshow enacts, they can afford to cover shipping costs. Of course, the main problem is that they're requiring people to send back the original headsculpt in the first place. FFS, just send people the replacement head, free of charge. These figures are expensive enough as it is.