Were it illegal, a good chunk of the public would be financially unable to fly. Thanks to differential pricing, a number of people are on the plane for little more than the incremental cost of carrying them, and many can't afford to fly, or fly as much, if they had to pay an equal share.

If you ended differential pricing, vendors would find a way to do it. After all, it's only sometimes they sell exactly the same thing for a different price. There's always some reasoning -- such as the Saturday night stay, or the change rules or whatever.

What you really want is to ban differential pricing that uses one difference as proxy for another (namely tolerance to pay.) Good luck trying to write a law to ban that! If you banned pricing based on fare rules they will just hunt for other proxies. You certainly can't stop them from changing the price based on what day it is (thus pricing higher for last minute purchase) or giving you a discount for buying a return trip. Go that far and you might as well have the government set the prices. In the regulated days we all paid a lot more for air travel, and planes were less full (ie. more wasteful from an environmental standpoint.)