The cost to JLC of stopping the production line and hand-picking the components you don't want assembled would be a lot higher than just allowing the production line to assemble them.
Back in the dark days of the 1960s and 1970s and early 1980s the New Zealand government (wrongly) wanted to improve the economy by employing people to assemble TVs and cars here. The Japanese manufacturers had to get someone to run around and pick one component from each bin and put them in a bag. They charged us more for the kitset than they would for a fully-assembled TV or car, because it cost them more to do.
And then we had to pay people to (inefficiently) assemble them, people buying TVs and cars had to pay a higher price than they otherwise would, and as a result had less money to spend on other things that might have gainfully employed the people who were working in those factories. In fact we would have been better off to import assembled products and pay those workers full salary to sit at home and do nothing -- and better off still if they found some other work (as they would, in time).
This was studied by Alan Gibbs, maybe as part of his M.A., Economics, or perhaps later when he was already working at one of those import substitution businesses.
Sidetracked, sorry ...