It's only part of the premise, not its entirety. A large part is a sense of justice, or ideology, that tempers the pragmatism, and acts as an instictual signal that a law might be harmful in the long term.
Compare it to the 'do no harm' in medicine: say a doctor kills a healthy patient, so that his organs can save five others. At first, many lives are saved, but when this becomes widespread, how long until no-one dares come near a hospital? Or how likely is a patient to admit to having suicidal thoughts, if it results in police going through their private home, confiscating guns, or even committing them for their own good?
Absolutist, 'ideological' positions such as do no harm, and doctor/lawyer-patient/client confidentiality, are important guards against unwanted second-order effects.
Compare it to the 'do no harm' in medicine: say a doctor kills a healthy patient, so that his organs can save five others. At first, many lives are saved, but when this becomes widespread, how long until no-one dares come near a hospital? Or how likely is a patient to admit to having suicidal thoughts, if it results in police going through their private home, confiscating guns, or even committing them for their own good?
Absolutist, 'ideological' positions such as do no harm, and doctor/lawyer-patient/client confidentiality, are important guards against unwanted second-order effects.