Traditional walled gardens protected the plants from high winds and frost, in fact they often create a warmer micro-climate as the brick walls release stored heat from the sun. They often sit alongside stately homes, where they would have provided vegetables, fruit, herbs and flowers for the household.
The same term has been borrowed for a more modern, geek-world use. It has come to mean a virtual environment where entrance/access is controlled. The best examples currently are facebook – which controls a person’s access and the provision of content, and apple’s operating system which limits developer and user access.
Sometimes the walled garden is created as a security measure, but most often it’s now a way of maximising profits. A supplier wants to keep you in their own environment as long as possible – that way you’re more likely to buy from them or be exposed to more of their ads for which they earn income.
However more cynical visitors may refer to the area as a “walled desert” particularly if the content within the garden is not as rich as the content outside.