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?
You are confusing the cartel's transactions with the non-cartel transactions. The later are the ones that get delay. The cartel's miner excludes them when adding a block solution to the block chain.