(Please do not cite Artifex v. Hancom. Special conditions prevailed in that case that do not apply, in general, to all software under a nonexclusive open source license.)
Generally speaking, under common law, in order for an agreement to have the force of a contract, there must be an offer, acceptance, and consideration. Since most people acquire open source through a download, there is no consideration (monetary value) being remitted to the author of the software, and hence the license does not have the force of a contract binding on the author. The license is thus a bare license, and the author has the right to revoke it at any time. Unfortunately that's how contract law works in common-law jurisdictions, and we are in for some nasty surprises once this actually does get tested in court.
Civil-law jurisdictions, for example Germany, do not necessarily require consideration in order for a contract to be binding. Open source licenses such as the GPL can be binding contracts in Germany.