Are questions that are US-specific on-topic on this site? (i.e., only relevant to academia in the United States)
(Do people want to see those kinds of questions tagged or marked in any particular way?)
US specific questions are fine. They're frustrating when a question doesn't indicate that it is US specific, when it in fact is. It is a sign of cultural and institutional arrogance not to indicate the culture of institutional-system for which your question or answer applies.
Ideally, if the site's traffic is high, system specific questions should be tagged to allow users to ignore or favourite tags.
Ideally, we might think about whether each particular question that appears to be system specific, is, in fact a useful opportunity to supply a full answer for all major systems.