To me, whether suing for trademark infringement is ethically supportable or not comes down to the purpose of trademarks: to prevent consumer confusion (consumers thinking they're buying the product of one company when they're actually buying the product of another company).
If a company is suing for trademark infringement, but there isn't any reasonable chance that consumers will be confused, then the lawsuit isn't legitimate -- it's just plain old bullying.
To me, whether suing for trademark infringement is ethically supportable or not comes down to the purpose of trademarks: to prevent consumer confusion (consumers thinking they're buying the product of one company when they're actually buying the product of another company).
If a company is suing for trademark infringement, but there isn't any reasonable chance that consumers will be confused, then the lawsuit isn't legitimate -- it's just plain old bullying.