The biggest issue with allowing the ai to have high APM is that it will inevitably learn optimal strategies that depend on that high APM, eg stalkers can take on far more immortals than we normally expect, and the AI will learn it this way, because the high APM allows a new stalker strategy (or rather, empowers an old one greatly) while not affecting immortals significantly. This also naturally means the AI leagues see a different game balance than the human leagues, leading to strategy divergence.

And then when you drop the APM limit, suddenly all the learned optimal ai strategies start falling apart, and the whole thing has to be relearned.

More annoyingly, there’s not much for human players to learn from innovative ai strategies that are based on inhuman accuracy of play (because we couldn’t possibly execute it).