Well no one asked them to open their threads, if they enjoy helping and sharing their knowledge good for them and good for us
Exactly. I
chose to open these threads. I've gotten help on many subjects and from many people here on DDF and I'm more than happy to help out others in my area of expertise. More importantly, I could chose when and what to answer; if I'm busy or not in the mood I could easily ignore it as long as I like.
But when a friend who I haven't spoken to in 10 years suddenly remembers my number and needs a camera NOW, or if someone wants me to take "just a few pictures" for their website (for free, of course), I'm definitely not inclined to help.
I think anyone in <insert field of choice> could relate to the feeling of being used sometimes.