Why should it be illegal for airlines to charge a fee for carry-on bags when they can charge a fee for checking bags?

I don't get it. No one likes to pay more for anything, but why should one particular way of collecting money from passengers be forbidden? If I'm getting on a plane with a bag and I could either check my bag or not, why should the cheaper option be the one that slows down other passengers in 3 separate places (the security line, getting on the plane, and getting off the plane)? Right now, the checked baggage fee has cost-conscious travelers dragging more bags on board. Why is it always the most annoying people who find champions in Congress?