Probably just a glitch. But then again, was it ever confirmed that Shippo's mother was a fox demon...?

Um, this is a *slight* mistranslation, I'm sure. In Shitoism (the belief most prevelant to the time of the series), there were many, many, many types of "spirits", which when translated into English were all lumped into "Demon".

Shintoism is extremely confusing to those who haven't studied it, so it's not a big suprise - The Japanese audience didn't even bat an eye at it, you can be sure.

Technically, only the Oni would have been what we commonly refer to as "demons", and the rest are a thousand varying shades of "nature spirits", and from a "technical" perspective (of an imaginary show ;), Inuyahsa would be considered Half-Oni (dogs/wolves where nearly unheard of back in the Warring States era, being imported from China only rarely, and hence "exotic" and likely "demonic"/Oni), whereas Shippo would be a lesser nature spirit, like... (damn, I forgot his name) - the Racoon spirit that associates with Miroku.

Neither Oni, nor spirits, had strength comparible to their size in Japanese mythology (re: the Kappa & Tengu were very feared, despite their deminuitive stature, and both of them were considered malevolent "spirits", not Oni.)

The primary determining factor, from what I can tell of Shintoism in what made an Oni an Oni rather than a complex nature spirit could be translated into the difference between "natural", and "supernatural". Spirits were often the most well known, or at least, easily explained - the more powerful and less understood (or foriegn) the entity was, the more likely it was to be termed "Oni".

As for whether or not it's pertainent to the story? Not really - mainly just as a subconscious trigger to identify Shippo as a "trusted" character - remember, the Tetsaiga is a spirit itself. ;)

It's not like the kind of "magical enchantment" that most westerners think of (ala D&D style, unless one was thinking archifact or rune item) - Tetsaiga can *choose* whom to enact it's defenses against and who not to.

In Japanese beliefs (where Shintoism met Bushido), when a blade was created, it's initial *intent* created it's spirit/personality, but once created, it had a spirit quite separate and distinct from it's creator... (as displayed in the episode when the other swordsmith [again, forgetting the name & ep - bad fan boy! LOL!] got possessed by the evil nature of the spirit that he made into the sword designed to destroy Tetsaiga...

In general as a matter of tradition, such "sword spirits" could only weilded by those bushi or samurai who had similar aims and purposes and often when they were "commissioned" they were bound to the purpose of those whom were destined to weild it's heart's truest desires. (hence why Sesshomaru had no problems picking up the blade which had possessed it's creator - it was originally made for him, and was mosty harmonious with his aims - it was the evil swordsmiths "taking it too far" and poisoning it with a will for Tetsaiga's destruction that caused all the negative backlash on him, because it was not harmonious with Sesshomaru's desires - he wanted to possess Tetsaiga - not see it destroyed)...

So, basically, since Inuyahsa has never really had the aim of running around and lopping off the heads of lesser nature spirits like Shippo, Tetsaiga would see no reason to give him (or any other of his kind) a jolt... (If you watch the series carefully, there are times when other malevolent *spirits* were able to make contact with Tetsaiga and not get zapped, either - but it was usually when Inuyasha had no reason to have a beef with them in the first place)
Malevolent Oni, on the otherhand... *ZOT*!!!

*whew* LOL! As you can see, Japanese Mythology is very complex - not for the faint of heart, but it is very facinating! But if you study it, it will ony deepen your appreciation for Inuyasha - as they took great pains to faithfully reproduce those beliefs in the series in great (albeit subtle) detail...

(unfortunately, alot simply gets "lost in translation") :S