It’s called toilet plume.
A toilet plume is the invisible to the naked eye, cloud-like dispersal of raw, infectious sewage particles as a result of flushing a toilet. Flush particles rapidly rise out of the bowl and several feet into the air after flushing. Aerosolized droplets can remain suspended in the air for tens of minutes between users, posing an inhalation risk. These invisible particles go on to coat and spread onto surfaces like floors and counters or objects like hand towels, bathmats, or even toothbrushes.
A cat fountain speeds up the process by exposing more surface area to the air. Lots of cats also appreciate moving water, based on instinctual understanding of the safety of running vs stagnant water.
Also, the cat bowl tastes like cat spit and this is unacceptable. The toilet gets freshened more often.