Why did the cops take a breath alcohol test on Diana?

Here, there are at least two reasons the breathalyzer is legal.

1. She was already under arrest. They have clearly detained her, handcuffing her. And they explicitly tell her she is under arrest for Assault and Public Intoxication. The test is a search incident to arrest.

2. She Agreed to the Breathalyzer. He asks, even if it's falsely implied that it's required, and she says okay.

The law allows for cops to trick you into consenting to a search. Many people do because it's convenient.

She may not have had to consent to it, but she did, making everything else moot.

Legal Reasonings:

Not sure what state this is in, but US law is very divided on this sort of thing. Some states have banned it, others allow it. Some municipalities do it at the local level. Some laws allow for requesting a breathalyzer for no reason, reasonable suspicion, or on probable cause. While a request can be done, or it being standard policy, it is not required to submit to it, except for 13 states that have "implied consent" laws for Licensed DRIVERS that punish them for refusing.

Well, those 13 states have some reorganizing to do. The recent Birchfield v. North Dakota 2016 Supreme Court Ruling:

Because the impact of breath tests on privacy is slight, and the need for BAC testing is great, the Fourth Amendment permits warrantless breath tests incident to arrests for drunk driving. Blood tests, however, are significantly more intrusive, and their reasona- bleness must be judged in light of the availability of the less invasive alternative of a breath test. Respondents have offered no satisfactory justification for demanding the more intrusive alternative without a warrant. In instances where blood tests might be preferable—e.g., where substances other than alcohol impair the driver’s ability to op- erate a car safely, or where the subject is unconscious—nothing pre- vents the police from seeking a warrant or from relying on the exi- gent circumstances exception if it applies. Because breath tests are significantly less intrusive than blood tests and in most cases amply serve law enforcement interests, a breath test, but not a blood test, may be administered as a search incident to a lawful arrest for drunk driving. No warrant is needed in this situation. Pp. 33–35.

Because the court always responds in the narrowest circumstances based on the case before them, they focus solely on drunk driving, but there is nothing there that limits it to just drunk driving. It relies on the search incident to arrest:

The search-incident-to-arrest doctrine has an ancient pedigree that predates the Nation’s founding, and no historical evidence suggests that the Fourth Amendment altered the permissible bounds of arrestee searches. The mere “fact of the lawful arrest” justifies “a full search of the person.” United States v. Robinson, 414 U. S. 218, 235.

While a cop generally needs your permission to search you, a judge approved warrant based on probable cause or probable cause in exigent circumstances (if not searching you will lead to destroyed evidence by the time the warrant is approved), If you have been arrested, that all goes out the window. They are now allowed to (reasonably) search you completely (cough cough cavity search). To take inventory, or to prevent contraband from entering a jail, assessing a prisoner for medical attention, searching for more evidence or whatever.

All US jurisdictions have implied consent laws, but they only apply to operation of vehicles. (The 13 could relate to criminal as opposed to pseudo-civil penalties, or could relate to something else such as impaired driving in other than a motor vehicle or the like.)

In all US jurisdictions, it is necessary for the police to show "probable cause" ("reasonable grounds" in Canada, not to be confused with "reasonable suspicion" in the US). Without probable cause, results of a test given under a Implied Consent requirement would be excluded.

source: "Guilty until proven innocent - impaired driving stops"

The film is fiction, but a chemical test can be taken with the consent of the suspect, or by warrant. There are also instances in which police will take a sample after arrest and hope the evidence is not excluded in court. The film was made prior to the Birchfield v. ND decision, so it was not entirely clear that such evidence would be automatically excluded.