You *really* just don't get it. It's supply and demand. By buying shark fin soup, you're adding demand. Yes, the shark is already dead, but by buying a bowl, you're enforcing to the seller that there is a market, and he goes through stock quicker, and in turn buys more. So more sharks will be killed in future because you ate a dead shark today.
If you ate a bowl of sharkfin soup once a month, that's 12 bowls a year. Say it takes one shark fin to make a bowls of soup - that's 4 more sharks (assuming 12 fins, 3 fins per shark) that will be killed in the next 12 months, JUST FOR YOU. Those sharks aren't dead yet. The guy you buy soup from (YOUR supplier) needs 12 more fins a year, so he buys 12 extra from HIS supplier. The guy selling him the fins (probably a wholesaler) now needs 12 more fins - so he puts an order in to HIS supplier - etc etc. It goes all the way back to the fishermen - who kill another 4 sharks to meet demand. Okay, so they're not going to go out one day and specifically kill 4 more sharks and say "these are for Collin", but at the end of the year, they needed 4 more sharks than they would if you weren't buying the soup. Now if you and a friend stopped eating soup, that would be 8 less sharks a year. 10 less people = 40 less sharks a year. 100 less people = 400 less sharks a year. Adds up quick doesn't it?