The reason is because the cartel needs to gradually consume the hashrate of the network, so it can delay the transactions of non-cartel customers who are on the blockchain. To force them to join the cartel or lose customers to the cartel.
Why?
You think they will create transactions that don't need to exist, just so that they can
not send them out to the rest of the network?
From the network point of view that is the same as the transactions not existing in the first place.
Why bother with all the hassle, just to pay yourselves transactions fees that could be totally avoided by doing offchain transactions in the first place?
They need to gain hashrate in order to withhold transactions.
They need to withhold transactions in order to gain hashrate.
Phase 3: Profit?