If Norton 360 installed Folding@home, would you call F@H a "cancer"? No, you wouldn't. The true cancer here is pre-installed antivirus software such as Norton.
F@H is using the energy for something useful. I don't view the creation of a digital speculative asset as useful in the same way I view scientific research (something terribly underfunded as-is)
Crypto is a cancer even when users voluntarily participate, but that's a separate discussion. It's incredibly inefficient and I distrust anyone downplaying the severity of fragmented commerce networks that individually use more energy than entire countries with millions of fungible flesh and blood humans.
Then it wouldn't be F@H, you'd be part of Norton's botnet. It's not just the AV companies, uTorrent had a controversy around the same thing. All about making a quick buck and crypto is enabling it by getting people to buy in to what is essentially a botnet