Can't understand specific problem of Bitcoin anonymity
I (as attacker) already know the owner of address B, don't I? why I
  need trap site?


You're right, if you sold the bitcoin and sent it to an address, and you know the identity of the person to whom it was sold (from KYC, for example), then you can associate that person that address. Since it is publicly verifiable on the blockchain, you don't need a trap site in order to see when that bitcoin is spent, but if you run a trap site, you can determine what they spent the Bitcoin on.