Unilateral promises themselves are not legally binding, and donations do not constitute a valid contract, as these lack consideration (or "causa," a related concept in civil law). It's also evident that intellectual property licenses should be considered contracts.
So, a license must be a valid contract that requires a mutual exchange of positive or negative promises.
While the most widely used permissive licenses (such as MIT or Apache 2.0) provide a valid consideration in terms of reputation and exposure because the original author information must be kept, some ultra-permissive licenses, such as Zero-Clause BSD do not provide such obligations from the licensee to the licensor (CC0 on the other hand provides a lengthy explanation on licensor's moral motivations, which probably constitutes a valid consideration).
However, there is a waiver of responsibility included, which is likely more of a disclaimer than a significant element of consideration.
It seems that in the case of Zero-Clause BSD, the licensor gets nothing from the licensee.
In this case, is there a valid, binding contract between the licensor and the licensee restricting the legal actions of the licensor?
