Because quite simply.
The reasoning they took money back from MY transactions to other people with MY money I sent to them from MY pocket is to cover MY debt. They took back MY money I spent. Thus they have cleared MY debt, with money that was mine. I sent it to someone else but if they take it back from them , they are stating that transaction is null and void, thus returning it to my account to cover my debt. Thus I should be credited. If I paid them at this point. Theyd MAKE 800.00 profit. Catch me here?