If you can make a shirt for $1 and sell it for $10, you can throw out literally half of your inventory and still make $5 per shirt.
Stated another way: you can total up the manufacturing cost of the shirts you destroyed ($50) and distributed evenly among the ones you sold (50/50=$1 each) and just add that to the cost of each shirt you sell when calculating profit. Same result.
Some people argue that the whole system is going against the objectives of recycling stuff but at least it's better than just burning it to get rid of it.