"If you’re driving or walking back into the US from Canada or Mexico, and you don’t have your passport with you, be prepared for some hassles. Although you won’t be denied entry to the United States, you will be taken out of line to be interviewed by US Customs and Border Patrol officers. Your return to the US will be delayed until the officials are satisfied that you are a US citizen."
https://passportinfo.com/blog/new-passport-fines-on-us-borde...
You can clearly be denied boarding a flight by an airline if you don't have the proper documentation.
Yes -- airlines are already fined significantly if they knowingly admit someone undocumented, with wrong documentation, etc by the destination immigration authorities, so it's in their best financial interest to have their own internal strict controls as to who can get on and who can't.