What % of incidents were first offenses? Unless it's a really low number, firing after first offense won't reduce the total number of incidents enough.
As a ridiculous hyperbole: assume a life in prison minimum sentence is established for speeding, on the first offense, and that it's actually enforced. I guarantee you that very quickly we'd have everyone driving under the posted speed limit at all times.
"If most offenses are first offenses" satisfies the predicate "Unless [% of first offenses] is a really low number"
The consequent "firing after a first offense won't help much" is consistent with "firing after first offense won't reduce the total number of incidents enough."