can you explain this in better words: "then it would always make sense to rent 51% of the capacity at market rates, earn the transaction fees, and also perform a double-spending attack."
Yes, but only if mining the coins via renting is cheaper than buying them outright.
If you're a miner, it makes sense to rent out your gear, because you get a guaranteed payment higher than you could make via mining.
For any purchase, there's a trail that leads to you through however you paid for it; for mining, the mined coins are totally disconnected from the hardware that mined the block and how you bought it.