From a legal point of view, it would be fairly easy to make a case that it's implied when you sell internet access.
From a business point of view, failure to provide DNS would appear no different from an outage to the vast majority of users. An ISP that didn't provide DNS would be out of business in very short order.
From a business point of view, failure to provide DNS would appear no different from an outage to the vast majority of users. An ISP that didn't provide DNS would be out of business in very short order.