Many of immigrants that came to America in the past did it to escape some sort of unfairness in their countries (such as religious persecutions, tyrannical government , etc)
Because of this, a lot of them actually value freedom and wanted to create a society where the past restrictions that they felt no longer exist.
That's being said, the practice of slavery was very common back then. In United States, those practice were propagated by European countries that colonize different part of north American territories.
For quite a long time, people with different view in United States s lived together. Until eventually, both of them clashed during the civil war. The people who valued freedom emerged victorious.