I'm not sure that really matters. I think their goal is to get as many people to see their comic as possible, through any free/cheap, and easy ways possible. The easiest way is to let people post images of your comic anywhere that they please, so as to get people to want to see more comics, prompting them to visit the site. The only visit that matters is the visit that generates legitimate site traffic. Rehosting makes the extra views free, hotlinks would only be considered cheap. Instead of spending money to advertise the comic, why not spend it by letting people advertise the comic for you. It makes for relatively cheap advertising. Of course that doesn't mean I think that artists should feel compelled to act like this, as I understand the importance of earning a living off of what you do. Maybe I'm wrong, but that's how I view things here.