Earlier quoted context omitted.
The point is if the two groups behave differently wrt risk then the trial is probably ruined. Maybe you're assuming the vaccine will be 100% effective; experts I've heard are saying to expect something like the flu shot.
Experts need to control to control expections. The flu shot like 50% effenceny is a possibility, and we know anti vaxxers latch onto that with confused statistics. The flu shot is poorly effective because the flu mutates though, covid is more stable so odds are that the vaccine will be more effective, but we are still playing odds of something we don't know, so it is best to assume the worse.
The inactivated polio vaccine, for example, does not prevent infection. It just prevents disease. That means that it cannot confer herd immunity (vaccinated people can act as asymptomatic carriers, leaving unvaccinated people at risk), but for most people who are vaccinated, it eliminates the most serious consequence of polio infection - paralysis. The SARS-CoV-2 vaccines might end up being the same, conferring protection from disease but not from infection, and even that would be important.