It's better to say that "Protectionism can be good for a country's economy". Every economically successful nation today, from the Asian tigers to the Western nations, practiced protectionism. The colonized world, the Third World, was forced to have no protections from capital flight, highly subsidized mega-corporations, etc.
Even if one could agree that a full circulation of goods, capital, labor and currency was good in theory, in practice that never happens. Everything from the existence of corporations to the ability of rich speculators to hoard their wealth and not turn it into any real investment to the fact that past and/or present subsidies are always involved means that the playing field isn't level. Peasants in Asia and Latin America can't compete with Western agribusiness, because Western agribusiness was and is subsidized.
Everyone who proposes reductions to protectionism never actually levels the playing field. They try to tilt it even more in favor of the already rich.