Assimilation had been taking place for more than 300 years without rules.
That's a clue ~
You're wrong about this ... my grandfather immigrated to the U.S. in 1902 through Ellis Island - there were laws, rules, procedures, and policy in place, including a health exam and quarantine.
The 1920's immigration rule modifications dealt with the numbers allowed in, and established the nationality quotas that remained in place until the Ted Kennedy revisions in 1965.