About 50% of the airfare you pay is often taxes. It does appear to be illegal for airlines to advertise fares that don’t include government taxes:
H.R. 4156 would require the United States Department of Transportation (DOT) to promulgate a rule related to how prices for certain types of air transportation are advertised. Under current policies, DOT requires that advertised prices for air travel reflect the total price per passenger to be paid by customers, including government-imposed taxes and fees. Under H.R. 4156, sellers of air transportation could instead advertise base airfares, provided that advertisements also disclose the amount of additional government-imposed taxes and fees that customers would pay.