Experience nullification if too higher leveled?

So, If you are the kind of person that have played several rpg games until now,
you may have noticed that some games have imposed that you will not
acquire experience if too much higher leveled than the enemy level,
but let's wonder, is that fair? Let's gather some thoughts.

The leveling is a progression in a game that allows you to go farther in it, and
face new challenges equally, but if you are overleveled than it, the combat
will be easier, includding a boss fight you are stuck on.

But when you are overleveled, some games likes to nullify the experience acquired
from the combat, which may be bad in case there is backtracking, since the character
would need to return to a low level area, the player would not be acquiring any
experience from his adventure, even though the character is so powerful that is
a overpowered compared to the enemies it's facing.

But on the other side of the coin, the character must grow stronger with the challenges
of around his level, or even higher, but still, the player need to gain something from
the adventure, other than loot.

There are also some games that lowers the experience acquired from defeating a
higher leveled enemy, which is stupid, since the battle is going to be tough, the
character should earn experience worth the effort.

But what If a developer wants to allow the player to acquire experience from
lower leveled monsters, what should that person do?
One of the things you should look forward to doing is make a experience table that
ascends in a way that if you are 10 or 15 levels higher than the enemy, the experience
is not that good to grind, meaning that you would take around 1000~2500 kills of that
same enemy to be able to achieve a new level, something that would be less if
you were facing a enemy of the character level.

The other would be adding multipliers to reduce experience received when facing lower leveled enemies, and increase when facing higher leveled enemies.

Well, that is all for my post about Experience Nullification when too higher leveled,
you decide wether it is fair or not, or necessary.