This is something which really annoys me as well.
Near to where I live, we have a nice big country park, called Cosmeston Lakes. This is a very popular place for dog walkers and walkers/runners/horse and bike riders alike. The staff at the lakes recently had a day where they went around the whole of the park, picking up full, discarded dog poo bags. They counted them up at the end and put posters on every single gateway, dog bin and stile they could stick them to.
They found over 300 bags :shock:
I'm also lucky enough to live very close to Cardiff Bay, and the Cardiff Bay Barrage. This is where our dog has most of his walks during the week, and it's the complete polar opposite. Instead of dog poo bags everywhere, people just don't bother to clean up the mess! The barrage is an area which is incredibly popular with both locals and tourists, and there is a large grassed area where you will often see kids playing and people sat having picnics during the better weather. There is absolutely no excuse for leaving dog mess lying about in a place like this and it should be picked up and disposed of at all times.
However, with an area like the country park, which has acres and acres of open fields and woodland, I really don't see the benefit of picking up the mess in a poo bag (unless it's on a path) and then disposing of it, even if it's disposed of in the proper bin. Just leave it where it is (so long as it's out of the way), and it'll soon work it's way down into the soil. It's more use there than it is on a rubbish tip somewhere.