It's not a baseless assumption that fitness trackers abuse access to people's private data. Maybe your particular fitness tracker doesn't today. That doesn't mean others don't.
And I am - for argument's sake - agreeing with the notion that privacy violation happens when the online service does something "wrong" with the data, not when the online service demands to keep their own copy, for their own uses, as a precondition to using the software.
History has shown that software that sends as much data home as possible can simply not be trusted with that data. Fitbit is one "innovation" or management team change away from further abusing the access to their user's data. I don't mean to coldly disregard the fact you trust your employer. But if I want an accurate assessment I can't care. Facebook employees trust Facebook.
My employer was being lumped in with a currently existing privacy violation. I won't argue much against hypothetical future possible privacy violations. I'll just point out that web-based email, your credit card company, and whoever installs software on your smart phone overshadow privacy implications associated with fitness trackers.
"Name, Address, Email, Carrier, Line Type, Secondary Phone, Age, Gender, Household Income, Marital Status, Presence of Children, Home Owner Status, Home Market Value, Length of Residence, High Net Worth, Occupation, Education Level, Twitter Handle & Followers, Facebook Profile & Followers, LinkedIn Profile"