First they have to know or infer with reasonable certainty that you have a fitness tracker to even attempt a further inference that you're not sharing the data because you're lazy. So if they somehow can profile you well enough to know that you already lost the privacy game.
Not at all; it's in their best interests to lump the "no data provided" bucket somewhere around the average, perhaps below as an incentive to provide the data. They don't have to know if you have a fitness tracker at all to enact such a policy.
