也 in classical Chinese as copula
Because it is a coupla.


  Confucius rhetorically asks: "Among territories (方) which are 60 to
  70, or (如) 50 to 60 li, where can you see (安見) one (者) that isn't a
  state (非邦也)?"


方六七十如五六十而非邦也 specifies a territory (方) which is about 60 to 70, or 50 to 60 li that hypothetically may not be a state (非邦也; not be is the function of the coupla). The idea is that such territories do not exist, because you cannot pick out any one (者) among the set of territories of these measurements which isn't a state.