Hey. Nations are made up. Citizenship is made up. Naturalization is made up. We have these things because this is the social contract we grew over hundreds of years.
The idea of an immigrant becoming a citizen is no less legitimate than the idea of someone being born a citizen. They are both and equally legal constructs that exist because things were worse without them.
If an immigrant can be denaturalized, then anyone the government wants to be rid of can be denaturalized.