If the lead car braked when there was no reason or danger, that driver might also be cited for recklessness or some such, but a rear-end collision is always an error by the rear driver.
Rear ending being the rear car's fault is a norm of the insurance industry that is used to settle disputes in the absence of other information. Furthermore, in the overwhelming majority of jurisdictions the government specifically does not establish "fault" for routine vehicle accidents (the government generally only decides matters of fault inside a court room) and the paperwork that comes with a police report of an accident usually says something along the lines of "we don't establish fault, just record the facts" prominently on it.
That said, yes, in theory you have to be alert, as the car in front of you can at any moment do something unexpected. (Eg. the driver falls asleep and crashes into the barrier.)