[nrmitchi]

More to the point, as-far-as-I-know this activity is monitored for, and using promotional credits for these purposes will invalidate your credits and you'll receive a large bill.

Source: A previous employer got a notification that our instances had been flagged as mining cryptocurrency, and they wanted to invalidate a low-six-figure amount of credits, and back-bill. Note that no cryptocurrency was actually being mined, and I don't know what heuristic they got caught in (although I have suspicions).