They aren’t “legal immigrants” just because they’ve been here a long time.
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They are legal residents, not citizens. To me a legal immigrant is someone who has completed the citizenship process.
For an adult immigrant to become a U.S. citizen, he or she must go through ‘the process of naturalization’. ... Be at least 18 years old at the time of filing the Application for Naturalization, Form N-400. Be lawfully admitted ‘permanent resident’ of the United States.
In order to qualify for naturalization, the law requires you to show that you have resided in the U.S. for a specific period of time and that, for all of that time period, you have been and continue to be “a person of good moral character.” ..........If you are still on probation or have not completely paid a fine, you will not qualify to naturalize. This includes any traffic tickets.....
.......... murder and other aggravated felonies, will automatically and permanently bar a person from ever being considered to have good moral character.If you have one of these on your record you will NEVER qualify for U.S. citizenship, no matter how long you wait.
If you apply for naturalization with a deportable conviction on your record—regardless of how much time has passed—USCIS may place you in removal proceedings. Most domestic violence convictions, including protective order violations, and nearly all convictions related to firearms and drugs, can make you deportable.
Immigration law provides that a crime can escape classification as a crime of moral turpitude if it is a “petty offense.” The petty offense exception applies if the penalty for the crime committed could never exceed one year of imprisonment, and if any time the person actually served in prison was less than six months.
Examples of petty offenses could include shoplifting, simple assault, or a DUI that did not involve driving without a license or damage to property or persons, depending on the law in your state.
‘Naturalization’ is the process by which U.S. citizenship is granted to a foreign citizen or national ‘after’he or she fulfills the requirements established by Congress in the Immigration and Nationality Act (INA)
https://www.nolo.com/legal-encyclopedia/crimes-that-will-make-immigrant-deportable.html
In legal terms, a naturalized person is no longer an “immigrant”. A green card holder, or “permanent resident” is the definition of “immigrant” under US law, and there is no requirement that such a person ever naturalizes. They can live their entire life as an immigrant if they wish.