This is a denial-of-service attack; whoever is doing this is not stealing coins, but is succeeding in preventing some transactions from confirming. Its important to note that DoS attacks do not affect peoples bitcoin wallets or funds.
If no one is gaining anything, then why are they doing this?
Not everything is for direct profit. Why do hackers DDOS charities? What is the point in that? Why do people spend countless hours ruining the work of others on wikipedia with stealth edits? Why do players grief other players in online games even to the point that it starts demanding a significant amount of their time?
Maybe the intent is just to cause as much chaos, bad press, and confusion in the Bitcoin space, or maybe it is just some ass who is bored and likes screwing around with people.