Long time lurker, I'm watching The Godfather part II (hence myusername, I just created the account and came up with that) and in one of the first scenes you can see tons and tons of italian inmigrants getting checked in an inmigrant inspection station in Ellis Island. My question is, why does the United States always have had that pro-inmigration policy?
What makes you think that? The US has long been seen as the "Land of Opportunity", but prior to those events (1860 to 1900-ish) there existed signs like the much-debated "No Irish Need Apply" and Chinese laborers built railroad infrastructure for two thirds the cost of their European and American counterparts, much to the competition's chagrin.
Immigrants have always upset the status quo and therefore inspired discontent no matter what country you care to speak of.
It didn't; there are actually quite a few examples of discrimination and hostility towards immigrants, like German and Irish And some Asian immigrants in the 1840s and the rise of the a Know-Nothing Party (http://www.realclearpolitics.com/articles/2015/02/18/immigration_and_the_rise__fall_of_the_know-nothing_party_125649.html) and quotas established with the Immigration Act of 1924 (https://history.state.gov/milestones/1921-1936/immigration-act)