The example they gave was about the comparative value of being vaccinated vs not. Since the population is vaccinated above the herd immunity rate for the MMR vaccine, then there is a perception by the individual that there is little to no personal value to recieving it given that herd immunity protects them. So in their mind, the risk of an adverse event outweighs the risk of becoming infected. It is rational. Then the question is on whether it is reasonable. For someone with a high likelihood of an adverse event (such as from a prior dose or existing condition) then this is also reasonable. If they don't have an increased likelihood, then maybe it's not reasonable.

Of course if we fall below herd immunity, then the cost/benefit changes. Rational people who previously elected to not be vaccinated (without higher risk factors) would be likely to then elect to be vaccinated, in theory.