But basically yes, ettiquete is totally subjective. What one person thinks is rude doesn't even cross the mind of the other person
That's right, so when in public it's better to be extra polite so you offend the least people.
Cutural difference would play a big part in that too. Like how in Japan it is offensive to blow your nose in public, so people stick tissues up their nose so they don't need to blow it! When I worked around lots of international students it seemed like some were really disrespectful, but then I read that interrupting someone who is serving someone else is expected in some asian countries, whereas here it is rude to interrupt someone if they are doing other work. It is suprising what is polite or rude in other cultures.
I also hate people eating food in a vulgar manner! I can't stand it when people eat in inappropriate places (particularly in meetings, or on public transport). But then I wonder if that's just because I am over sensitive to it?