This angle works for many businesses and users. There is a second side to it that the analogy fails to consider. If you open a safe you inmediately see what's in there.
Any unusual thing is easy to spot.
And you can only use a safe to store things. I remember one case in which the secret services of a country contacted one University complaining of somebody breaking in their systems from there. When followed, the chain pointed to some other countries' secret services and an embarrassing situation for the University. Now think how would your small business feel if, say the DFA said that some colombian cartel was using your business to clear their laundry or sell their stock. And how would you explain that you have nothing to do with that, and that you don't care if they use your computers for that since you don't store your sensitive data in that safe.