The carnist claim here is that sheep don't get hurt in shearing, so it can't be unethical
Sheep are part of animal agriculture, they are in factory farms with the rest of the animals living in confined squalor and are routinely slashed by automated shears, and are abused and terrorized like all the others once they meet their end.
Go ahead and look up "mulesing", which is a procedure harmful to the animals regardless of if it is claimed to be for their health.
It's paying money for animal agriculture, and funding the torture of animals, thereby increasing the profitability of taking a sheep and turning it into meat.