By induction - repeated exposure to ideas increases their staying power in the mind - see advertising.
If one accepts the premise that fascist ideas spread by exposure, then one way to limit their spread is to limit said exposure.
Sidenote: I'm increasingly frustrated with this increase in veneration of scientific studies, as if studies are the only way or even the best way of attaining or approximating truth. I would like to see more acceptance of lived experience as admissible evidence in matters of debate.
How do we know that? Has a study been done? Are there surveys that track it over time?
Racists beliefs have decreased over the past 50 years. I haven't seen any evidence it's spiked since the advent of the internet.
More visible? Sure. But more visible doesn't mean more people hold those beliefs.