I find it absolutely not acceptable for a customer to expect automatic credits for down time. If you want a credit call in state your reason and you might get a credit in fact its highly likely you will get a credit.
Why, exactly? There's a slippery-slope argument which favours a strategy of automatically refunding customers for downtime on a pro-rata basis... if I pay a monthly fee for best-effort service, how much downtime can I expect before that service is considered "not provided"? Is it a few minutes a month? A day? What if the ISP decides that its all too hard to provide me with service this month? An automatic credit for downtime makes a lot of sense for the ISP... firstly, it cuts the cost of having to have a call centre to answer and deal with requests for refunds. Secondly, it gives the customer trust that the ISP takes downtime seriously and will refund for periods in which it didn't provide a service. Finally, it allows the customer to know that the provider treats all customers fairly and discourages "squeaky wheel" behaviour.