I don't think it's bcrypt that does it. It's the underlying implementation of bcrypt chooses to do it for the users. Or does the actual bcrypt standard says implementation has to generate one for user?
It seems that bcrypt does not specify that salts are randomly generated, but it does specify that the salt is stored with the hash. The “Bcrypt Algorithm” section of the paper at https://www.usenix.org/legacy/publications/library/proceedin... shows salts to be an input to the algorithm, and also describes them being concatenated with the hash. Random generation of the salt must be just a de-facto standard for bcrypt APIs. It’s true that that feature is orthogonal to the hashing function, and so libraries for other hashing algorithms could easily copy it.
I think the part concat with salt is required. But generating it FOR the user is orthogonal to the hashing function. But I will be surprise if the paper actually says it is required to generate one - because that shows the cleverness of the inventors, but also a potential weakness (implementation may be using weak random source, though counter-argument is that user supplied salt can be weak too).
