Why do we assume zero mean noise in sensor data?
The brutally honest answer here is: The noise is considered zero-mean because that's what the author decided to do. Without looking deeper into the signal model employed, it's impossible to answer. 

However, for many systems this makes a lot of sense physically, since the processes leading to a noise realization are very often zero-mean in nature. For example, the thermal noise over any resistor should be zero-mean, simply because, well, that's how these random fluctuations work.