I don't care if it's stealing or not, the dog deserves better than her current 'owners' do for her - and I use that term loosely for those wastes of oxygen. My family and I have 'stolen' my lazy neighbor's pregnant cats to care for them until they had their kittens, then found the kittens homes and had the mothers spayed. My two sibling kitties were from one of these litters. In fact, all the pets I've ever owned were animals I picked up out of the backyard. Sorry, but if the same cat or dog shows up in your yard more than a couple times, chances are the owners are worthless scum and do not deserve to have the animals. This would be the only time I'd advocate stealing because another life depends on it.
I hope you get to keep Maggie, because she deserves some spoiling and a good home. If not, I would advise taking her in and telling the Lazy Idjits you have not seen her if they come looking. But judging by their past and current attitude, I doub't they'd even notice she was gone.