It's to do with bandwidth theft I think
With the image tag thing, every time the image is loaded, the outside server has to use its bandwidth to display it.
This costs money for them, so they would want to avoid it.
Inline linking (also known as hotlinking, leeching, piggy-backing, direct linking, offsite image grabs and bandwidth theft) is the use of a linked object, often an image, from one site into a web page belonging to a second site. The second site is said to have an inline link to the site where the object is located.