The current LP Pledge is required by section 5.1 of the bylaws:
Members of the Party shall be those persons who have certified in writing that they oppose the initiation of force to achieve political or social goals.
Some radicals in the LP try to use the Pledge to castigate members who disagree with anarcholibertarianism, but this distorts the original meaning of the Pledge. LP founder David Nolan says the Pledge was instituted to protect the party from possible accusations that the LP seeks violent overthrow of the U.S. government. When many (perhaps most) moderates took a pledge in joining the Libertarian Party, they were joining a party that they believe had competently named itself and thus had consciously decided not to call itself the Anarchist Party. As they signed up to be a party activist and support the party's political actions, it didn't seem very strange that the party might ask them to pledge tactical non-violence so as to give the party plausible deniability for anything destructive that the new members might try to do in its name. The oppressively legalistic atmosphere of the modern nanny state makes such silly CYA certifications all too common.
Many such moderates were aware of how anarchists use the phrase "initiation of force", but it was always in the context of absolute abstinence for any purpose. The LP Pledge qualifies the usual anarchist formula with the above vague language about "political or social goals". If the LP had intended an oath of absolute fealty to the Zero Aggression Principle, it would have used a normal and unqualified statement of it. One could reasonably conclude that these "political or social goals" must be a reference to the goals involved in the step they as Pledgers were taking: adopting the Party's goals as their own.
The 1972 Platform seemed to deliberately make room for minarchism that involves a non-zero amount of coercion. The Pledge was instituted while that minarchist 1972 Platform was in effect. Melinda Pillsbury-Foster reports that the Pledge was already in force when she joined in 1973, but it's not mentioned in the 1972 Bylaws. D. Frank Robinson (1972 Bylaws/Rules Chair) says "it was on the membership forms for as long as I can remember". He writes: "As you may gather from the history of the time, we assumed that we were under surveillance - remember Hoover had just died. We never promised to be pacifist. So I guess I agree with David Nolan [that the Pledge was a renunciation of revolution]."
Scott Bieser wrote on alt.politics.libertarian in 1994 that the Pledge was on LP membership forms in 1972, and relays the standard Nolan rationale. Dave Hollinden cites Nolan's rationale in a 1993 posting on the same forum. Robert Bickford mentions that rationale in a 1992 posting on alt.politics.elections.
The Pledge has a curious qualification regarding "political or social goals" that is not in the prior libertarian art for force-initiation language. Rothbard wrote about the Pledge in his "lost" 1977 paper Toward A Strategy For Libertarian Social Change (quoted by Ed Crane here): "anyone can become a party member simply by signing a vague (and non-enforceable) pledge, and once a member he cannot be expelled". The 1976 OKLP Bylaws adds "economic" to the current phrase "social and political goals". Thus the phrase we currently have was apparently not understood by Libertarians in the 1970s to be a clear, unambiguous, and airtight endorsement of an absolutist principle of political theory — or else why extend or clarify the scope of the Pledge?