It's because people may not be purchasing a new ticket, but wish to change their return flight. It's only the outbound flight that is subject to the advance purchase restrictions.

For example, you purchase an L fare with a 21-day advance requirement. By definition, this will be a return ticket. However, after arriving at your destination you decide to go home a day early. Three (or perhaps more) conditions will have to be met. First, you are still meeting the minimum stay requirement (i.e. Saturday night stay.) Secondly, there must be space available in the fare category (i.e. "L" in this example.) And finally, the fare must allow changes. (Most will, upon payment of a change fee, but there are some fares that are simply not changeable.)

There may be other restrictions as well, such as time of day (after 7pm only) or day of week (Monday-Thursday only.) There can even be other weird restrictions, but these are the main ones.

So as long as you stay within the fare rules, you may be able to grab that L seat tomorrow right now, by simply by paying the change fee.