This type of vehement negative reaction has surprised some people, but as an amateur student of history I am much less surprised. Xenophobia and ingroup-outgroup mistrust and hatred have been the historical norm for millennia. What is amazing when you really think about it is how far humanity has come in just the last few generations, and how the people who make these awful bigoted statements are becoming more and more marginalized. Not long ago awful statements like these on Twitter were rallying cries for elections and popular movements, and printed on the front pages of newspapers, and circulated nationally on pamphlets...not the subject of ridicule and condemnation. One does not have to look far into the historical record to find numerous examples of anti-Irish, anti-Polish, anti-Jewish, and most notably anti-African-American bigotry.
Anti-immigrant propaganda cartoon from 1896