Why is it logical to assume that the hydrogen ion concentration in an aqueous solution of a strong monoprotic acid equals the molarity of the acid?

Strong acids are those that dissociate completely into their ions. So the dissociation equation for a strong acid is:

HA → H⁺ + A⁻

It is visible from the equation that the number of moles of hydrogen ions released is equivalent to the number of moles of acid. For a given volume,

[HA] → [H]⁺ + [A]⁻

Thus, the assumption is logical and fairly accurate