There is a reason for this rule, and it is the same for a KO that goes OOB beyond the sideline too. The NFL wants KO's to be returned. That is why they moved the kicking tee back a number of years ago. Attempting to use the sideline to defend field position on a KO is something the rules comittee does not want to see. They want to keep the kick off return play remain an exciting one. Kick near the sidelines... run the risk of getting penalized.
There is risk in what Leon Washington did too... once he plants a foot OOB, he has to remain OOB, or get a penalty, he is not allowed to return from being OOB.
A little known rule, but perhaps not as awful as some people think it is. The NFL wants the KO to go where a return is possible. That means away from the sideline. Very smart play by Washington.