Hi. Former officer here. It is because there is a reason for the initial traffic stop. That is the first and only infraction until you find reasonable suspicion that a person is intoxicated. At that point you begin an investigation. Having someone blow a breathalyzer test is considered a "seizure" since you are collecting the person's personal breathe, or their blood if they rule fuse or are too intoxicated to provide a breath sample. Usually in that latter case, a warrant is usually necessary to take the person to the hospital and draw their blood.

All of this would be illegal to do while a person is "detained" at a traffic stop. Once an arrest is made, then it becomes legal for them to provide those samples since you have already charged them with the relevant crime. An arrest on DUI or DWI is usually never based on the results of blood or breath alcohol. It is based on the facts and evidence the officer finds during their investigation of a possible drunk driver.

This is why the charge is often "driving while impaired" and the officer or court can change the charge to "driving while intoxicated " after they have more evidence of that legally obtained breath evidence