I think the literal translation is getting obscured.
1 per person would be correct. 1 per account is different.
A family plan would involve min 2 people. Both people would need to be present (thus the 1 per person rule would still apply) Say myself and my wife are in line and we would both be in line at the same time thus ONE PER PERSON rule would be in effect. Unless she wanted to be difficult and say "I think I'm gonna go to the end of the line because I like it in here and want to make you wait" Otherwise, I would be first setting up the prenup and getting things in order and she would be 'next' in line.
It's more a courtesy rule than anything else. It's already going to be a hectic day and this is to help make things run smoother to avoid one person holding up the line who is 'claiming' to get a family plan and try to secure 2 or more iPhones at one time and they're spouses or whoever just can't be there.
Besides is it worth getting all worked up without knowing the facts? We translating a phrase and blowing it up to something that more than likely is not the case.