I don't think it is the theory of "why buy the cow when you can get the milk for free", but I think it is more that they want to give their visitors fresh, good quality content to look at (because if the LL visitors know that it the type of stuff they are going to get by going to thaqt particular LL, then they will keep coming back), plus better quality makes
your visitor think that they are going to get that same type of quality if they join. I probably wouldn't buy if the photos I was looking at were very low in quality.