The surface tension of water makes it want to minimize contact with the air
In microgravity, that means forming into a sphere, or in this case into the cylinder and minimizing the exposure to air
It is not gravity holding it in the bottom, it is surface tension
This video was from a class session they were giving to grade school students, who had the same objects in their classroom
The students floated the ping pong ball on the water, then the astronauts showed how they could push it down and it would stay there
Then they pulled it off the (velcro'd) surface and spun it around, again showing they are in microgravity.
https://youtu.be/0qEzdxLls5k 