Fair use applies to humans and the things they do (including AI). It is not something that applies to algorithms in themselves. AI's are not people, the people who use them are people and fair use may or may not apply to the things they do depending on the circumstances of whatever it is they do. The agent is always the human not the machine.
True; consider the "it" in my question ("If I make a spam detector network, why should it not be covered by fair use?") as "my making (and usage) of the network".
