Simple!

Because GOL accepts credit cards (at least it accepts Amex) registered at non-Brasilian addresses. i.e. I can book a flight from my desk in London and access normal airfares.

TAM adopts the policy "milk the gringo", insisting that reservations from overseas are made in higher fare buckets through its "international" website. The Brasil call centre also refuses payment with foreign credit cards.

For Webjet I went through their call centre to pay by credit card. Once you get used to Brasilian operators not understanding an Anglo-Portuguese accent, it works