I thought this too, but apparently there are no federal regulations other than reimbursement for the direct costs.
"Airlines are not required to provide passengers with money or other compensation for costs that fall outside of the cancelled airline ticket and fees tied directly to the airline ticket (such as baggage fees, seat upgrades, etc.) when flights are cancelled." [1]
"Each airline has its own policies about what it will do for customers on bumped or cancelled flights. There are no federal requirements." [2]
However, there are rules around "bumping" passengers on a flight.
"An airline is required to compensate you after involuntarily bumping you from an oversold flight in certain situations. However, there are many situations where you are not entitled to compensation." [3]
[1] https://www.transportation.gov/individuals/aviation-consumer...
[2] https://www.faa.gov/faq/what-are-policies-bumped-or-cancelle...
[3] https://www.transportation.gov/individuals/aviation-consumer...
https://www.reddit.com/r/SouthwestAirlines/comments/zw6upo/h...