There is a lot of confusion over what etiquette is. The Collins dictionary (I wanted OED, but it's $300 for a year subscription...no) defines it as
a set of customs and rules for polite behavior
I think of it like the law. Where I'm from, EVERYONE goes 5 miles over the speed limit on city streets and more like 10 over on the freeway, to the extent that if you're going 25mph instead of 30 people will be annoyed. This is regional - I know other areas go faster or slower, and on Highway 5 between SF and LA, for example, if you're not going 90 (20 miles over the limit), you're going too slow. But the cops don't accept "but everyone was doing it!" as an excuse because you broke the rules.
That's like etiquette. You can choose to ignore it, maybe even the majority of people in your area do ignore it, but there is no basis for the argument that it's an opinion or that it's subjective. It's a set of social rules, and if someone tells that that what you're doing is poor etiquette, that's an objective fact. Luckily, etiquette is not the law, and you can break it without a fine (I know I have!), but for the love of god, recognize that you're breaking it instead of trying to argue that etiquette is just a frame of mind.