I believe you are making an incorrect assumption that these botnet nodes are actually relaying on behalf of the network. I've not seen any reason to believe this is correct. Rather than just act as normal clients of the Tor network - placing extreme load on existing relays.
In fact, this botnet appears to be basically breaking Tor with many node operators reporting that their relays cannot keep up. The Tor developers recently started developing code to prioritise the more efficient NTOR handshake over the older protocol, and because the botnet runs older code people who upgrade to the latest code (once they are finished) should take priority over the botnet traffic. Until the botnet also upgrades, of course.
To make it worse, when a circuit fails to build because of overloaded relays, Tor retries. I'm not sure there's any kind of exponential backoff. Thus the network goes into a death spiral in which clients constantly try to build circuits and fail, placing even more load on the already overloaded system and making it impossible to recover.
Unfortunately we may be looking at the end of Tor here, at least temporarily. The botnet operator doesn't seem to realise what's happening, otherwise they'd be backing off. Tor is effectively experiencing a massive, global, accidental denial of service attack by this botnet. Many relays don't have enough CPU power to weather the circuit storms. It will be very interesting to see what the Tor developers do next - they don't have any effective way to fight off this botnet because almost by design they can't detect or centrally control the network. They practically have to ask nicely for the operators to go away.