I understand your point, but I disagree slightly or vehemently, depending on what part is most important to you.
To elaborate:
I agree completely that if the members of a group are all on the same page with regards to loot rules, there should be no room for drama to be caused, the exception begin due to honest mistakes which do happen, although not commonly.
I utterly disagree with the notion that it's the responsibility of the members who already know, and adhere to, the established social convention to preemptively inform any newcomer(s) of said conventions.
The very point of a social convention is to not have to reiterate it, again and again. (To quote Wikipedia, just as one example: "A convention is a set of agreed, stipulated, or generally accepted standards, norms, social norms, or criteria, often taking the form of a custom.") Note the past tense. Thus, the responsibility for learning about any social convention in effect falls primarily on the individual who is not yet familiar with it.
This can be done either by learning about them before placing one's self in a situation requiring the knowledge, or by asking about them right at the beginning of such a situation, or by adjusting one's behavior when informed about the error in one's actions during such a situation.
When I visit another country, I don't presume that what is custom in my own country is also custom in the other country. I try to learn before travelling what I need to know in order to co-exist peacefully with the people I'm going to visit. If I still make mistakes, and I am informed of my error, I adjust my behavior accordingly.
What I don't do, is expect my hosts to educate me on their own accord. The responsibility to become educated is mine, and mine alone. If I refuse to become educated, that is my problem and not one of my hosts, and it is I who will suffer the consequences, not them.
I apply the same approach to my online gaming, and it tends to work well.
In summary, there is only a minor adjustment needed to the statement of yours that I quoted above, in order for me to agree completely with it, and you. It would be interesting to see what your objections to my adjustment is, if you have any.
Here goes:
If a group-member who is unsure about the social convention regarding loot rules would take the time to ask about it, rather than just assume their personal idea to be it, there should be no room for drama to be caused.