CMC is not a police-type authority. It simply takes the information it receives and displays it. So basically when scam exchanges such as coinsbit and p2pb2b show they are trading US$ BILLIONS a week when they really have near zero traffic and near zero customer and near zero trade - CMC just takes their word for it.
One thing to note though is the manner in which CMC does exclude certain exchanges when looking at coins trade volumes. For example, here we can see the MNX (Minexcoin) scammers on HitBTC being excluded from the volumes because they claim to have buyers at $0.33 when the price being traded is around $0.09
https://coinmarketcap.com/currencies/minexcoin/#markets