LiQiCE,
That just depends on what rescue you work with. Each has it's own rules. They decide whether or not they want to risk their dogs life sending it to a home where they have to decide whether the person will really do what they say and keep the dog on a leash or in a fenced in dog park. Everyone has their own opinion. All you can do is ask. After all, it is THEIR dog you want them to give you.
Sometimes people just freak me out, but in the end you just have to laugh it off. You wouldn't go meet a dog that was "free to good home" in the paper, and then go psycho on the people if they decided they didn't want their dog going to a home where the people said they would feed it only table food (or any other million things you could talk about). You'd NEVER think about throwing a tantrum at someones house. But for some reason, a lot of people see a rescue as like a retail company or something. And if they get denied, they throw a fit "at the counter" like the rescue has the motto "the customer is always right" or something. When in the end, these rescue dogs live a FAMILY life in a Family environment and the dog DOES belong to that rescue, the rescue is it's family.
Sorry I kinda got a little 'out there' with scenario's. I don't mean that you are throwing a fit or anything, just pointing out extreme scene's that happen daily in the life of a rescue. My point is simply the fact that just because the dog is up for adoption, does not mean it is right for your home. No one knows the dog better than the people that live with it. Would you feel comfortable leaving your own dog with a perfect stranger that has no yard, and your dog is used to having a yard, and you want the best home you can find for your own dog?
I would suggest if there is only 1 boxer rescue in your area, and they turn you down because of their "no-fence" policy, maybe you could offer to join the rescue. It would give them a chance to get to know you and vice versa. There are a million other things in rescue to be done besides fostering dogs. In the mean time, they may (will) become full at some point and a shelter dog may fit your lifestyle.
You might even become close enough friends with them that they will cry on your shoulder about the foster dog they loved and recouperated and cared for, the one they let go to a home without a fenced in yard, the one that the people SWORE they NEVER let the dog off leash, but that was the ONE that was killed by a car, and that was the ONE reason why they will never bend their rule again.