Yeah, but the power delivery mechanics are different right?
At least when we are talking about the rear straight hand.
In tai chi the proper technique is to put all your weight from the back foot to the front foot when you deliver the blow. They avoid being "double weighted".
Also many teachers advocate creating a straight line from head to heel. One guy I know from another forum calls this straight line "a lightning rod" that helps deliver together with the weight shift the power from the ground to the hands.
So when they punch they look like this.
Sinister, according to what I have read here in sherdog, teaches his hard right quite differently.
I guess there are many ways to skin a cat.
Of course, the reason these delivery mechanics and stance in tai chi, have taken this form might also has to do with the fact that Tai chi is a mixed martial art, so they serve various different purposes.