[Q] Can an attorney licensed in one of the 50 states of United States practice U.S. immigration law anywhere in the USA?

Yes, an attorney licensed in any of 50 states of the United States can advise and practice U.S. immigration cases in all 50 states and in US. territories. This is because U.S. immigration law is "federal" law and not state law. To give you an example, let's say you are married to a US citizen spouse, and you want to get divorced but still want to file a separate I-829 application in the state (let's say Virginia) you reside. As long as the attorney is licensed in any state, the attorney can accept your I-829 immigration case, but unless the attorney is licensed in the state of Virginia, the attorney will not be able to accept your divorce case. This is because divorce case taking place in Virginia is governed by Virginia law, while I-829 immigration case is governed by U.S. federal government and therefore falls under "federal" law. Other federal law areas are: Patent, bankruptcy and maritime laws.

This is why it's good to practice U.S. immigration law, because if you are licensed in one state, you can move to any of the 50 states and practice U.S. immigration law, without having to waste many weeks or months studying for and passing the Bar Exam in the new state. You have to study for and pass the new Bar if you are an attorney specializing in divorce law or even corporate law. The only bad thing about being a U.S. immigration lawyer is that you have to deal with USCIS and consular officers. Who knows, maybe the USCIS and consular officers feel the same way. :) In all seriousness, one good thing about being an immigration attorney is being able to "help" people, but one negative aspect is having to deal with all bureaucracies and the inefficient system.