I think the reasoning is, they have two vaccines, one that in rare cases causes this one that doesn't. Even if the liklihood of the side effect is very low and the severity is also pretty low (its generally not life threatening or anything), if you have two vaccines, one that causes the side effect and one that doesn't, why would you use the one that does, regardless of how insignificant the risk is in absolute terms.

The comparison isn't moderna vs no vaccine (in which case moderna would win the risk trade-off). The comparison is moderna vs pfizer.