“Data from the accelerometer, gyroscope and magnetometers could be used to monitor the way your wrist moves, whether you’re tapping in your pin at a cash point, or entering your Facebook password on your phone. There are a wide range of scenarios where your wearable could be compromising your security.”
Yeah, it’s unlikely, but if it could happen, the fitness tracker companies might want to secure the devices a bit better. In the mean time, just wear it on your non-dominant wrist and use the other one to enter any personal information. Simple enough!