I always thought that getting that first set of customers that will take you to product/market fit was going to be easy. What with the Twitterverse, Facebook and all and sundry. But let me tell you, from experience, it is DAMN HARD!
I guess the true test of whether or not this is 'unethical' is whether those people that signed up felt scammed after using the service.
If I were one of those people that got that email, sure I might feel a bit 'weird' that they presented themselves as 'Jill D' and not AirBnB, but after going through what I have been doing the last few weeks - trying to get customers - I can't say I would be upset.