what's the point exactly giving away your work for free ?
i can understand the fun in showing pics around, imagine a camera club where on a big table everyone show their best prints in A4 or A3 ... great ...they can look, they can touch, but they can't steal it ... but on the web they can and they do and they will !

I can partly answer this.
I've been in several Flickr conversations from people asking about my watermark, so I've asked them why they're so happy to let their pics be used for free. There have been various answers, but in general:
1. It's mostly wildlife images, and people imagine that they'll be used for editorial/educational/conservation charities, and usually have indicated that they should not be used commercially, and fondly imagine people will honour that restriction. They're perfectly happy to allow education/editorial/charity use for an attribution - 'paying back'.
2. They dislike agency 'standards' which don't correspond with their own aesthetic, particularly with regards to natural  lighting (again, wildlife photos). They see captive animals on agencies labelled as 'wild', and many, many misnamed species, and have no respect for the integrity of the non-specialist agencies.
3 They don't care about pixel-perfection, and prefer to achieve interesting compositions or capture interesting action/behaviour and submit to exhibitions etc where content is what matters.
4. They are working full time and don't want to have to spend time captioning and keywording.
5. They don't photograph all that much and can't see much point in uploading a few photos each month, with corresponding low sales, then all the hassle of registering self-employed for tax for such a small income. Alternatively, they spend hours/months/years photographing the life-cycle of one particular species. Neither of these would make it possible to submit to a specialist agency, which require large numbers of a variety of species, regularly.

These are the main five. Don't shoot the messenger.