We could always discuss the FAA "common purpose" rule
It is extremely strict compared to the UK one, and I am sure it is widely disregarded in the USA.
On the ANO, I think the CAA have done the best they can while preserving PPL cost sharing.
To take one case: if the owner of a flying school rents one of the school's planes and cost shares it 75% (3 passengers), he is actually paying less than his 25% because he recovers a part of his rental via salary+dividends from the school. The CAA confirmed to me in 2003 that this is legal.