The point is to break through any blacklisting and forcibly plaster their URL into your memory. If you blacklist them, the friend request lets them keep sending you tells.
They're not fishing for someone who wants to buy gil today. They're getting their name/URL plastered into as many heads as they can so it'll be remembered. Just like radio commercials that repeat a phone number or URL over and over and over and barely mention the service they're selling. You're not going to rush home and call that number, but hear it enough and it gets stuck in your head, so if you do happen to find yourself needing/wanting the service, you think of them.
It doesn't matter if they annoy Bob today as long as Bob remembers them six months from now when he's gotten frustrated enough to want to buy gil.