Secondary enforcement

Many American states have the notion of secondary traffic offenses: the police may not pull one over for committing them but if stopped for some primary reason, like speeding, then secondary charges may follow. It depends on the state but secondary offenses are acts such as using a cellphone or not wearing a seatbelt.

I find the concept bizarre. I don't know if the theory is that being pulled over constitutes some kind of seizure under the Fourth Amendment or if the legislature really did not want to pass, say, a seatbelt law so making it difficult to enforce was the only way to get enough votes at all, or what. I am curious to know what the thinking really is and the degree to which it makes sense. I am not aware of an analog in English or Scottish road traffic law.