Yeah but. I somehow doubt that people leave a service over a single explainable outage. From my personal experience, I move when the outage is the straw that broke the camel's back - when I have been unhappy with the service but have lacked the time and urgency to do something about it, and then they are down and I'm thinking "Well, time I moved off here anyway".
So in that respect, I think I am fair game to a "Switch to our service!" email and I don't see how that is unethical - happy customers don't move just because they get an email during an untypical service interruption. Dropbox could go down for the rest of the day, and I could get all the competitor emails they want to send me, and I am not going to switch. I did switch registrars during an outage, but that is because I had been less than ecstatic about them and just needed a kick.
It depends. Rinkydink startups may not, but if a down service resulted in a nasty conversation with your biggest customer, then you probably should consider switching or diversifying
So in that respect, I think I am fair game to a "Switch to our service!" email and I don't see how that is unethical - happy customers don't move just because they get an email during an untypical service interruption. Dropbox could go down for the rest of the day, and I could get all the competitor emails they want to send me, and I am not going to switch. I did switch registrars during an outage, but that is because I had been less than ecstatic about them and just needed a kick.