So, the AoA indication is a feature that can be activated for a fee (which most likely exists to cover costs of modifying the configuration from the base config

So, the AoA indication is a feature that can be activated for a fee (which most likely exists to cover costs of modifying the configuration from the base configuration, so imagine as an analogy a $2 fee for some really minor change to a car you ordered). The reason this is an option is that it requires additional training for the pilots, some airlines already have this training (American Airlines is one example) and theres no enabling it and then not training the pilots on it. As many have noted, AoA is not commonly used in non-military piloting in the United States, so its no surprise its not commonly ordered. I am also under the impression that the AoA Disagree indication was new on the 737 MAX, I haven't checked the 737 NG manuals for it.