"I'm not ignorant of this issue, it just wasn't the point of this article."
It was downplayed far too much. It's a big issue, which raises the moral point of whether these apps are stealing from websites. Heck, it has to fake REFERER information to do it, which should be an indication that this app isn't on the up and up.
"is any such client a "thief"?"
If comic creators are going through extra lengths to protect their documents, such as hotlinking bans, that should remove all doubt. A hotlink ban is like saying "I don't give permission for you to directly access these documents". Whether accessing them though trickery and fakery can technically be called "stealing" is something that is a debatable issue; whether it is a copyright issue and copyright violation is not so debatable.
You raise some good questions. The bottom line is, the content providers don't want to be nailed with the bill from these kinds of clients. Hypothetically, let's say 100% of the comic readers used these clients. That would mean that all of the bandwidth would be footed by the comic creator, without a cent received through advertising support. A lot of comics would go down in a hurry.
All because people are too lazy to bookmark.