Fasting is an obligation upon every Muslims who is able to do it in Ramadan, but people that are sick or on a journey have the permission to not fast. Besides, there are children upon whom fasting is not an obligation yet and people who are too old to have the ability to fast, and there are also non-Muslim citizens. Therefore, there is no reason to forbid people from keeping the restaurants and other eating places open in Ramadan for their use. What is forbidden is eating and drinking, not selling them.
The owners of these enterprises would not be responsible for their customers who do not fast and eat out without proper excuse, since nobody can be forced to fast or questioned about fasting or about their religions. Therefore, the income gained out of these services is lawful (halaal).