A host has many "rights" - you must remember you're using THEIR services. While I wouldn't personally recommend pulling an account on ONE complaint, it is within their rights to do that I guess.
We normally have a few complaints and traces etc that we do to find out how many emails were sent out, and then act accordingly.
Many of the quick suspensions or terminations without any interraction are performed by smaller hosts that don't want to have to deal with the battle on behalf of the client. Pulling after ONE complaint is not something that should ever be done, but if you're getting complaints from ISPs, SpamCop and can trace the message on the server, then a suspension is in order until you contact the client. Who knows, maybe the client's site was COMPROMISED and it wasn't their fault for the spamming. You can then help them upgrade their software and make their site more secure to prevent the action in the future.
When it comes to things like hacking, or phishing sites etc - we suspend first, THEN ask questions.