It affects American businesses and therefore affects the American economy.

For instance, let's say Brazil's telcos started throttling their citizens' access to Netflix, but not to Braziliflix, a hypothetical Brazilian version of Netflix. That could prevent Netflix from gaining traction in Brazil. Meanwhile, Braziliflix would be free to compete on an even playing field for customers in the US.

Cool, that means consumers in the US will have two viable alternatives, Netflix and Braziliflix, so there will be competition and people will enjoy better level of service. And the two companies will try to out-innovate the other, hiring engineers and lowering prices. Sounds like economic gain to me.

Meanwhile, consumers in Brazil enjoy shitty services and exorbitant prices...

While that scenario is certainly better for American consumers, it's not better for Netflix. Both companies will have to innovate and compete in America, but Braziliflix will essentially get the Brazilian market for free.

Protecting American consumers is the FCC's job. They can't be expected to prevent protectionism in other countries.

That's correct. Note that I never said it was the FCC's job to prevent protectionism in other countries. I was simply answering deciplex's question of how net neutrality could harm the American economy if it were only enforced in the United States.