User-agent discrimination is tolerable when it's Joe Webmaster doing it out of ignorance. It is not acceptable if it is being used by a company leveraging their dominant position in one market to gain an advantage over its competitors in another market. It's not acceptable even if it's not said company's expressed intent to do so but merely a "happy accident" that is getting "overlooked".

Indeed, even for those who require a round of mental gymnastics before they concede that monopolies are, like, "bad" or whatever, GP points out precisely how this would constitute "consumer harm".