I know this isn't a historical answer I'm about to give, but I'd be sort of surprised if there were a historical answer for this regardless.

I think it's simply because that's where people would grip the pipe when they smoke opium, and that's what your fist would look like when doing it. People wrap their fingers around the end of the pipe around the bowl when smoking elongated pipes like this, so it's not unlikely that the artist wanted to capture that visual and implement it into the design of the pipe. When someone smokes that pipe, they'd wrap their fingers around the fist in a similar fashion.

And the fist isn't really a clenched/punching fist, if you look closely you can see that the fingers wrap around the bowl and sort of cradle it (rather than the fingers being clenched and the bowl is just above the fist). The proportions just don't work for it to be a clenched fist. But it perfectly represents the shape your hand takes when it wraps around the bowl while you're smoking. So it's likely just a decorative aesthetic that mimics the way you hold the pipe while smoking.

Again, I know this wasn't a proper historical post, but I would be flabbergasted if there was some sort of historical significance to that design.

I wouldn't be surprised if it was just a stylistic choice. You'll note that the bowl is shaped like a poppy seed pod, where they get the opium.

I've read pretty extensively on this topic, and while those types of pipes are fairly common among genuine opium pipes, I'm not aware of any particular significance to the fist shape. It seems like a common design choice when making a pipe of that length/configuration.

Here are the best books I've come across on the subject:

The Arts of an Addiction, Carlos Armero and Ben Rapaport - this is the best work on the pipes and accessories themselves. It has lots of pictures of very intricate and detailed items, but what sets it apart from the typical coffee-table book on the subject is its archaeological and historical perspective.

Opium Culture, Peter Lee - a general overview of the subject of opium smoking, including its history, practice, and effects.

The Social Life of Opium in China, Zheng Yangwen - this is an extremely detailed and thorough history of opium use in China, with a lot of primary sources. It provides a much clearer perspective on the subject than most Western histories, many of which are swayed a bit by the colonialist and missionary perspectives that were dominant during the heyday of opium smoking.

Good luck, and feel free to PM me if you have other questions.