First of all, it's terrible that she was singled out for what is obviously political retribution and we should all be ashamed of our country for allowing what is very clearly an attempt to chill and shut down any form of criticism. It's extremely inappropriate and we need to reform the system to make abuses like this forbidden. We must have a rule of law that applies equally to every individual and actively resist arbitrary applications of government authority regardless of the context.

With that said, in almost every other country in the world this scenario wouldn't have been a result of individual retribution, but just a normal procedure. The US is unique in that it vaguely allows visitors to publicly engage in political speech and protest for things opposed to the government's theory of what is in our country's interests, even taking positions that are directly opposed to the government or supporting groups that actively seek to hurt or kill the citizens of that country. If she engaged in this behavior in Germany or France or Spain or Japan she would have been deported without any real debate, and potentially would have been guilty of a serious crime (because lots of speech is criminal in these countries, and there are political and historical claims are illegal to express at all).

The key distinction is that, for example, Germany makes these rules explicit in the terms of the visa and is not arbitrary in their application. The issue here is not that it's 'unconstitutional' or 'wrong' to restrict the activities of foreign visitors. The issue here is that this was arbitrary and politically motivated arrest and detention, which cannot ever be allowed in a free society.