I don't think anyone at the airport cares about anyone's immigration status for internal flights. Following this reasoning "don't fly, it's not safe", flying to Vegas to get married would be unsafe as well. I fly frequently (once/twice a month) in the US and abroad, have audible foreign accent, but never, ever in my 25 years here had my immigration status checked before or after flying. My fiancee is a scholar/researcher, she's been to the US many times before (about 20 times), on B1 initially, and then later on ESTA, flew internally across the country (CA to MA and back) and never had a single problem either.
But let's not focus on the risks, we will manage these just fine. The risks are not the reason I started this thread.
All I want to know is -- is going to US VI directly (for example, flying from CLT to STT) treated as exiting the US and then re-entering for the immigration purposes? If yes, then we obviously can't do it on her K-1 visa.