I've checked into this, and pretty much without fail it would go against our Acceptable Use Policy as the background miners don't often make it clear that they are mining; this would be considered malware and would be grounds for immediate and permanent suspension. Even in the cases where it was made clear to the customer, if it interfered with their system in any way, shape, or form (such as if it was consuming all of their CPU or resources), it would also violate the AUP and be grounds for suspension.
Short version: Don't, there's better ways to make money than hijacking someone's CPU and trying to mine bitcoin/ether/today's-cryptocurrency-of-the-day behind the scenes.