I had a similar reaction the first time I saw that scene! However, the bizzare nature of this piece of equipment also shows up in Is There In Truth No Beauty?
When they first go down the corridor to Kollos' quarters, the flowerpot is nowhere to be seen:
When Marvick runs out of the cabin after opening the box, it is suddenly there, by the red line:
But when Kirk and party show up, the flowerpot has moved, so that the clod-footed guards don't knock it over:
My theory? It's some sort of portable wall structure strengthening device. That would explain why we've seen it all over the ship, as well as being distinctly portable!