I wonder what the total value to the Brazilian economy is to keep Visa/MasterCard's cut of so many transactions in Brazil instead of being siphoned offshore.

It's not just that. It's also about - one fine day a nation waking up to Visa/MC "disconnecting" that nation. Why? Well, maybe the mothership nation's leader just felt like it. Who the hell knows. The point is - these nations don't want to find that out. Almost all of these countries have faced such sanctions, blocking, control etc in one way or the other at some point.