We follow a 3 warning process as stated in the client service agreement. it is not good customer service to take a site down on the first non payment. Sometimes they forget to change their card or they have other things that they are doing. Either way, giving them a warning save you from having to deal with upset customers when they call wondering why their site is down.
I have to agree that-basically things come up some times. Sometimes people genuinely have an emergency that means that for a week or two they can't pay, or a credit card expires. Being able to work with people helps to keep customers loyal to you.