Okay, this thread caught my eye since I'm a martial-arts geek, and I have to pretty much concur with what has been said above. There certainly ARE quite a few different techniques that can be used with sai, but they are challenging to learn. I have no idea if the D&D rules suggest a learning penalty or whatever, but my two cents would be that this is along the lines of the cultural/regional suggestion someone else made, ie: depending on the level of "realism" you're attempting to portray, make it harder to learn or offer bonuses to the people who are more likely to know it.
Uhm, two cents on the technique thing: holding the sai in the "defensive" position with the main prong along the forearm is often referred to as the "closed" position and the reverse, held like a standard knife, is the "open" position. Sai are usually used as a paired weapon, mostly with one in one position and one in the other, to present both offense and defense. They can be easily flipped to the alternate position, allowing the wielder to switch their dominant hand to attack or defend as needed, surprise an opponent by suddenly slternating the technique being used by either hand, etc.
Also, keep in mind that most "ninja" weapons were originally farm implements or walking sticks/staves, because the ruling class forbade the commoners from bearing weapons. This let the powerful remain in power, until the commoners figured out how to use their everyday items as weapons. The sai did indeed start life as a pitchfork, the nunchuck or three-piece staff was a threshing flail, a quarterstaff or jo stick was obviously a hiking aid, etc. etc. So, sai were originally 'pointy', but not bladed, because the blades were what was forbidden by the ruling class. The farmers quickly figured out how to use their pronged pitchforks to
break the ruling class' blades...
(oh, and another historical note of interest is the ninja "Art of invisibility" thing: Hollywood would have us believe the ninja could hide in plain sight by somehow remaining unseen, etc. etc., but the this actually referred to the fact that no one ever
saw a ninja assassin, they were only known to have been present once the deed was done. Now really, how effective is an assassin that is seen coming? That's why it's part of the rogue class, obviously. The use of common farm implements as deadly weapons meant that
anyone could have been a ninja, since at least 80% of the populace were commoners/farmers. Food for thought.

)