Some of them certainly do use that sort of language in their promo material; on the Jawbone site, for example, one of the lines is "Make fitness a habit. Wear the motivation you need on your wrist to improve your days and nights."
Even if they didn't, I suspect a lot of people might buy them with that intention - "I know I should walk more. Maybe if I get a gadget that tells me how far I've gone, that'll motivate me to keep up with the target"
I know I've certainly thought like that. Usually before concluding that, actually, I live 50 metres from a decent pub, and that's quite far enough, thank you very much.