She did not really appreciate my answer since after I answered she hung up on me.
I explained to her that legally (not morally or ethically which is separate from legality) the fact that the cops never told her why she was being arrested did not matter. “While it is assuredly good police practice to inform a person of the reason for his arrest at the time he is taken into custody, [the Supreme Court] have never held that to be constitutionally required.” Devenpeck v. Alford, 543 U.S. 146, 155 (2004).
In Devenpeck, the Court rejected a requirement that probable cause for an arrest must be measured by reference to the offense that the officer named at the time of arrest. Such a rule was improper because it hinged on the subjective belief of the officer, and because it would have “perverse” consequences:
the predictable consequence of a rule limiting the probable-cause inquiry to offenses closely related to (and supported by the same facts as) those identified by the arresting officer is not, as respondent contends, that officers will cease making sham arrests on the hope that such arrests will later be validated, but rather that officers will cease providing reasons for arrest. And even if this option were to be foreclosed by adoption of a statutory or constitutional requirement, officers would simply give every reason for which probable cause could conceivably exist. Id.
In theory, the police do not decide if there was probable cause for charges to be issued. That determination is made by the prosecutor. Notice to a defendant of those charges is given at a probable cause hearing, ordinarily within 48 hours of their arrest. See County of Riverside v. McLaughlin, 500 U.S. 44 (1991); Gerstein v. Pugh, 420 U.S. 103, 105(1975). However, the Wisconsin court has left the 48 hour rule largely a toothless tiger. State v. Koch, 175 Wis. 2d 684, 499 N.W.2d 152(1993) (shifting the burden to the arrested individual to prove that probable cause determination was delayed unreasonably).
Also it might be helpful to point out a similarly widespread but mistaken belief that officers must read arrestees their Miranda rights. There is no right to be read Miranda rights on arrest. Miranda merely impacts what statements are admissible in response to police questioning of a person in custody after arrest or its functional equivalent. If the police do not want to question the person, they are free not to read the suspect Miranda rights. See generally Chavez v. Martinez, 538 U.S. 760 (2003).