The side-snap kick (yoko geri keage) is a well-known Shotoism. We do the side-snap kick at several places in several kata, but other styles all do a front-snap kick instead. There is no doubt that their way is the original: our kata were changed.
Why? The side-snap kick can be used for a few applications, but it isn't all that practical. What was really going on?
I have a theory, and it is the only one I have encountered to explain this change. See what you think.
Historical fighters kept their feet on (or very near) the ground, with their knees turned in to close the groin and protect both the knees and the testicles.
Sport karate outlawed kicks below the waist. This led to the high kicks that bloomed suddenly in the 1930s.
The side-snap kick is the practical counter to the new high kicks. You opponent wheels a roundhouse kick to your head. You lean away and side-snap kick his testicles. It is lightning fast and deadly, and is the only practical application of side-snap I have been able to find.
Or it could just be that side-snap kicks were flashier and showier than front-snap kicks, so Funakoshi made the substitution. There were many such substitutions during the period when katas were transformed from textbooks into dances.
But however it came about, the appearance of side-snap kick in our kata is a corruption of the original technique. When you demonstrate kata applications, show people the front-snap kick instead.