Nylabones are, at the moment, the only safe chew for unsupervised use for Logan. He can't have antlers or recreational bones because he's too hard of a chewer and already broke one enormous molar on an antler. He can't have the hardest of the nylabones, either, so he gets a step own from those. Anything softer isn't safe for him, because he'll chew chunks off and swallow them (he once puked up a piece of rubber bone that had to have been in him for several weeks if not several months).
Gavroche won't touch Nylabones save for the very softest ones, and since Logan can't have those, I don't get them, since I don't want him to have access to them.