To get the hotel to fax you the guest list, you'd either have to A) find a very gullible employee and/or B) Impersonate a motel 6 or an authority agent.
Having them fax you the list isn't illegal. If you impersonated someone else to do so, and the police got involved, they might tack on some fraud charges. If you pretended to be an authority figure to have them do it, then I'm pretty sure it could be a felony, and the proper authorities WOULD get involved in that case.
It's much more fun just to call guests WHILE they're at the hotel and impersonate the front desk, causing them to flip out while still at the hotel.
I've actually had hotels give out my information several times, which leads me to believe that hotel employees don't understand the meaning of sensitive information.