Ethically speaking I'd say it's completely okay to monitor the former, whether that service is a plumber or a babysitter, and whether the monitoring is done with a camera or your own human eyes. Those service personnel shouldn't be expecting privacy while they provide services, anyway.
Ethically speaking I'd say it's wrong and a violation of privacy to monitor the latter, regardless of whether it is done with camera or eyes, without prior warning. This is because providing a place of residence implies providing some level of privacy. Forget cameras, you shouldn't generally enter a tenant's place of residence even with your human eyes, without letting them know in advance (e.g. at the very least by knocking).
