There's nothing wrong with a license, which is essentially a contract. You simply can't enforce your contract on third parties unless they agree to the terms. If I sell you software but the contract says that you agree not to give it to third parties and you must sacrifice a chicken each full moon, that's compatible with Libertarianism. However, if you violate our contract and give the software to a third party, I can't enforce the terms of that contract on him or her.