I see blogs that post whatever random photos they’ve found from all over the Internet, with the disclaimer that everything they post is “assumed to be in the public domain.”
That’s like a bar owner saying that all drinks they serve are “assumed to be non-alcoholic.”
I mean, selling booze without a license isn’t the worst crime in the world. But don’t be like, “What? How did this beer get in your Samuel Adams bottle?”
Brian D. Buckley 
I see three things that contribute to this:
1. this is a really a legal issue and even simplified legal explanations can be dense and hard to understand.
2. this is an extension of the culture that social media has encouraged. I’m sure all those super EULAs contain legally correct instructions for users, but social media encourages us to share whatever with whoever. the casual blogger likely isn’t even aware of the actual legal issues involved with publishing certain content.
3. the internet makes if easy, in some cases, to use content illegally.
without better ways to enforce it and a culture that is totally aware of the legal repercussions, I don’t see this changing.
should this matter to the casual blogger tho? what’s the real consequence to a copyright owner if a small blogger posts an image that is seen on their page 200 times a year, half of which are by crawlers? to me, this kind of feels like getting a traffic ticket for going 101km/h in a 100km/h zone.
As I said, it’s not the act itself that bothers me so much, it’s the absurd disclaimer about it.
I don’t mind so much if a driver goes slightly over the speed limit, or even if they’re unclear on traffic laws. But when a driver covers up their own speedometer and says “I always assume I’m going the right speed,” it’s hard to take ’em seriously.
For me it depends more on the effort behind the disclaimer.
UK law includes provisions that permit an image to be used without breaching copyright if sufficient and reasonable steps have been taken by the user to identify the copyright holder without success. So, if someone does reverse image searches, checks them all for attribution, follows any “obtained from” that come from those results, &c. and can’t find the copyright holder, then they could legitimately put a disclaimer.
And someone who was aware of the need to check but not a legal expert might use “public domain” rather than a more accurate term.
However. I suspect some disclaimers are I found this image so I’m going to assume I can just use it until the owner tells me I can’t.