Someone being fired and replaced by someone else is not inherently wrong, and that doesn't change at all whether they're an immigrant or not. What you seem to be complaining about is the practice of companies doing so specifically so that they can get a cheaper worker who can be abused more, or doing so when the original employee had done nothing wrong; however, phrasing this as "an H1B visa holder is working a job that should be occupied by a domestic worker" is the absolute wrong way to go about it, because this makes it sound as though an immigrant having a job is fundamentally less worthwhile than a citizen having it.

This is a government policy failure, allowing misuse of the H1B program. Be angry at the company and the government, but don't blame the worker who is inherently in a worse position than the domestic worker, both in and out of work, for taking the job.