Then I suspect that if you are a landlord or property manager, you have a bad problem with insomnia. The bottom line is that there are million things that tenants do on a regular basis that you would not approve of, and you have no means of doing anything about. The least of your worries should be a homebrewer with a heatstick.
And who do you think the insurance company will go after if something did happen and it was found to be the fault of that DIY heatstick, etc?