Protection is plunder indeed because it is the privilege to operate scarcity through legislation, creating dearth in order to abuse the buyer.
Complete Works, Volume 1, pages 334 to 386 (in French)
Frédéric Bastiat recognises in his conclusion that if he compares protectionism to robbery, it is not yet the case for most of his fellow countrymen. However, he highlights the fact that making something legal is not sufficient to make it legitimate. As a proof of this, he appeals to the past when wars were conducted in order to conquer and the victors were glorified. These wars of conquest were not glorified in the 19th century anymore thanks to the evolution of society.
Thus, it is the evolution of society that will allow to recognise protectionism for what it is one day, i.e. a cancer for society at the same level as theft. Unfortunately, we are not there yet.
I also note that a lot of people still deceive themselves in justifying some actions through their legal status. However, this does not necessary make them legitimate.