Sure, external constraints aren't always apparent to the public. But in this particular case, the Brazilian airlines can't claim that because visitors to Brazil

Sure, external constraints aren't always apparent to the public. But in this particular case, the Brazilian airlines can't claim that because visitors to Brazil can buy airline tickets through other interfaces (from travel agents or at airline ticket counters [1]). So their back-end systems are perfectly capable of selling tickets without demanding a Brazilian identity number (the CPF number). This problem exists only with their web interface. [1] http://brazilsense.com/index.php?title=Booking_a_domestic_flight_within_Brazil#The_easiest_ways_to_buy_a_domestic_flight_in_Brazil http://brazilsense.com/index.php?title=Booking_a_domestic_fl...