"why is this a problem if the publisher doesn't want a third party to use their work without their permission, or at all?"
This misses the point. People when given a choice about something they don't fully understand will generally be conservative and protective. It would be very easy for the default to become locked-down for resources that haven't really had this fully considered for them. Which, if we're being honest, will be most of the web.
It's also not so much about using a third party's work - the more valuable thing is being able to see how something works.
If you want to share something, then share it. If you don't want to, then don't. Nothing is forcing people to share their content.
DRM is saying you want to have your cake and eat it too. It's sort of like they're sharing content with you, but not really. You have to consume it the way they want to you to consume it. That's trying to control the experience, not the content, and I don't see any reason content creators have that right.
If they don't want to let the content loose on the world, then they don't have to.
This misses the point. People when given a choice about something they don't fully understand will generally be conservative and protective. It would be very easy for the default to become locked-down for resources that haven't really had this fully considered for them. Which, if we're being honest, will be most of the web.
It's also not so much about using a third party's work - the more valuable thing is being able to see how something works.