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 no hackers DDOS charities? Why do players grief other players in online games? 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 likely screwing around with people.