Do you think locking your front door treats everyone else in the entire world as a criminal?
"the end result is that we all wind up being treated as potential thieves"
We all are potential thieves from the perspective of anyone who doesn't know us; and if you have a valuable good, it's a far more reasonable (and profitable) assessment than "no one is a potential thief." It's the same stance you take when you design a website: "everyone is a potential hacker."
A potential thief, however, is not an actual thief, and to repeat what I said above, I don't believe DRM treats users as actual thieves: it treats them as potential thieves until they've authenticated and as honest users afterward. If you have to authenticate annoyingly often, it's a usability problem, but it doesn't change the essential dynamic.
"DRM starts from the premise that all readers are slimeballs and thieves."
Such a dire pronouncement doesn't logically follow. DRM starts from the premise that there exist many slimeballs and thieves. Next, that we should thwart as many as we can with technology, even at the inconvenience of paying customers, presumably yielding greater profit than in the absence of DRM.
The first assumption's undeniably true; the second is open to much debate. But "all readers are scum" is a needlessly cynical attitude to read into it.