Rgarding ur First query - I think it is based on Sec 26 -
Luk AOP is considered as a separate assessee distinct from its members.
The term "AOP" is of wider meaning & covers "Co -owernership" as well.
Ya u r rite Sec 26 is mandatory becos of the use of the word "shall". Further this point was also comfirmed by Honbl HC of Calcutta.
Sec 26 is a section which is beneficial to assessee. So Bhai lets be satisfied with the fact that Law has given a mandatory force to a beneficial section..........:))
Regarding ur second query - Difference between AOP & BOI -
Now I will show u magic -Differnce will be clear on its own just expand these terms -
Association of Persons
Body of Individuals
Now "Persons" as defined u/s 2 includes - Individual, Fiirm, Company,etc .......even AOP..........:))
"Individuals" not defined but I guess not required.................:))