If the garden were a large enough area where the dogs had enough space to play, I would say it's not a big deal if the owners didn't take them on walks. However, it sounds like they don't really pay that much attention to the animals and don't spend any time with them. Do the dogs display any aggressive behavior? That's usually a result of owner neglect.
Here in Charlotte, North Carolina, where space is ample, I have a large fenced in area, and I have to admit that there are times when I have to let my two pooches out for maybe twenty minutes or so while I do other things instead of taking them on walks. They stay inside during the day, because it's safe and it gets very hot in the summer. That's a much better deal than what the German Shepard next door gets. He's tied up year round in the middle of a junk yard.
Your four legged neighbors might not have the best life in the world, but unless they show physical signs of abuse and neglect, such as injuries, disease or malnutrition, it would probably be difficult to get the local authorities involved. I would follow the advice of some of the other people on this thread and try to befriend the neighbors, and their dogs, and offer to walk and spend time with them so they'll have a better quality of life.
Peanut and Levi say: "WOOF!"
