I think it's more complicated than that (though no-one here wants my several pages of academic writing with footnotes which analyse this!).
1. People don't actually obey the law because they don't know precisely what it says. Instead they obey what they think the law is (which is influenced by what they think it ought to be).
2. Copyright law has been negotiated over the years by commercial producers and users of works like photos, so it is completely ill-adapted for the kinds of casual or private use that are nowadays made online. Before the internet, private uses were just ignored as being trivial.
3. There is now a strong social norm that stuff online should be shared, strengthened by all the sharing tools built into online technology.
4. If someone acting non-commercially did find out what copyright law really demands and tried to comply, they'd discover that the pricing structure for use of works is set for commercial uses (see point 2). For example, the photo on a local charity fund-raising page would probably be priced at, say, £10 if the photographer were asked personally.
None of this is going to be fixed for decades for many reasons, including because the "right" answer is really hard to see!
Doesn't this also come back to more fundamental philosophy on the right to own anything - from physical property to IP - fundamentally the pattern of finding images and just using them online is basically theft, if someone on here builds a table then they have a fairly understood moral and legal right to enjoyment of that table for their own use - or to sell it (and in our capitalist society, to set any price they wish, the potential purchaser having the choice to buy or not as they prefer). If I pop over to their workshop and take that table, then I am depriving them of that right = theft.
One of the reasons people have a different moral compass with regards to photos is that they see it as a very low cost path to creation - just pushing a button, and as they have little commercial value for the photos they produce on their mobile phones, so they extrapolate that to there being no commercial value anywhere - ignoring the fact that a professional photographer may have tens of thousands of pounds worth of kit, and with some photos they may have been on location for months to get the right shot - the cost of one shot could be multiple thousands of pounds.
This is not helped by big business skewing IP and the patterns of behaviour - on the one side they are very keen to secure swathes of IP for themselves and are very strong in their pursuit of those who may breach that... on the the other side, they see huge commercial advantage in providing tools which completely blast a hole through the norm in how IP is respected - pinterest relies on building commercial value in exploiting other people's IP, facebook, twitter, instagram all allow the spread of other's IP - youtube is perhaps one of the few platforms which actively tries to stop misuse of someone else's IP...
It is frustrating when someone gets caught, but ignorance isn't in itself a defence to breaches of the law, and it is probably a case of pay up and move on...
In answer to specific points here:
- 2 - I don't think it is ill-adapted, there is no basis for causal or 'private use' if it is on the internet then it is not private, nor is it usually casual, effectively what is being said here is that pre-internet private mis-use wasn't spotted so was okay and that the internet hasn't adapted to allow that to continue - surely pre-internet it was still as wrong as it is now, but it is now easier and more obvious (and has more impact) being in a more public arena.
- 3 - as above the social norm is promoted by companies who make money from it - doesn't make it right - society's job isn't to roll with trends changing because they are pushed that way by big commerce / public opinion, it is to establish what is right and provide the framework for that... those who are happy to steal other people's photos may well also be fast to shout if their content is then taken by someone else...
- 4 - it is not tricky to deal with photography at a cheap level - the quality of output from a standard mobile phone is stunning (I recently sold all my pro DSLRs and now use an iphone for 99% of my photography) - so they can take their own photos - there are stock libraries with millions of photos available at far cheaper price points than the more traditional Getty / Magnum type licensing deals - they can approach photographers they find on Flickr / elsewhere and offer to pay a nominal licence fee...
ultimately it is laziness / ignorance / entitlement which leads people to just use photos they find - having owned a stock library in the past, and 30+ years as a professional photgrapher and c. 20 years running a design company perhaps gives me a different perspective, but I don't believe that it is hard, it is only when people are caught out that they will change their habits...