[ARCHIVE ANSWER]

Police can perform two legal searches after every arrest: 1) Search Incident to Arrest.
This means after a citizen is arrested, their person and auto can be searched. 2) Inventory
Search. The intention of this search is to log all property in the citizen’s vehicle to protect the citizen’s property while in jail.

The truth, it is common for police to arrest a person for the traffic violation so they can "search incident to arrest" and "inventory search.”
The courts have ruled if contraband is located during either two searches, it can be used as evidence.
Police will threaten to arrest and inventory the vehicle upon hearing a refusal. If this does not scare the motorist into giving consent, they will arrest and inventory because the motorist refused consent. This is illegal and can be beaten in court, but it is usually not beaten because the officers' ill intentions have to be proven.
This method, along with K-9 false alerts, is another underhanded, hard-to-prove technique police use to trample citizens' 4th amendment rights.
Not all officers commit this illegal act, but the majority do.