You don't like reading the whole article.
From the article "But most often she sends cash with friends traveling to El Salvador. Legal channels for the international transfer of money charge 10 to 20 percent of the amount of money sent. It's a high price "
Anyway, a high tax does what high taxes always do, high taxes will cause high tax evasion and a drop in revenue.
I guess you did not read about supply side economics either.
Oh by the way, remittances include transfers by legal immigrants too.
Mexican banks charge a hefty tax on the remittances at their end, by the way.