Yes, this is legal:1) An employer may set any policies it wants in regards to cell phones: employers generally have wide latitude or discretion to set policies about communications, use of devices, etc.2) Employees can be forced to agree to company policies on pain of termination (i.e. if you don't agree with your employer's policies, leaving employment is your only option).3) If employees agreed to a policy fining them for certain actions or violations, then those fines are legal--even if the employees only agreed in order to avoid termination.