"Assimilate" doesn't equal "throw away your own culture", it means become a member of society able to live and communicate in this society as well as the natives do. Assimilation isn't only good for the society, it is also good for the immigrant him/herself. The immigrant is free to hold on to their native culture, and a lot do; you will find, for example, that a lot of Korean CEOs in the US try to bring in some of the Korean work ethics in their companies, and a lot of Chinese immigrants read Chinese books, study Chinese philosophical teachings, teach their kids to do the same and so on. At the same time, they don't get stuck in the circle of other immigrants from the same culture, which hinders their ability to become a full member of the new society: you won't get far in NYC, for example, if you just sit in the immigrant ghetto on Brighton Beach, get jobs offered by other Russian-speaking people in that same ghetto - you probably don't even need to learn English to live there. You hardly even live in America this way, more like you live in Soviet Union, just geographically on the other side of the ocean.
If you just temporarily visit a country, then I can understand the lack of desire to assimilate. But if you plan to live there for many years, including permanently, then assimilation is pretty much a requirement for being able to take the opportunities the new place provides. So, by ensuring that immigrants learn the language, you make sure that they will have a quality of life closer to that of the native population - than if you encourage them to be stuck in Chinatowns where barely anyone speaks English and cares about doing anything to improve their quality of life.