BBM. In some cases, as in this one, there simply is not enough proof to hold someone on charges for what LE suspects the person has done. To prematurally charge someone with insufficient evidence can jeopardize the case. So if there is another reason for which the person can be held, and LE has strong reason to believe a person is a danger to others out there, they can and do exercise their right to arrest the person for charges that may not always be pursued. Yes, many times, if not most all of the time, police do not actively seek those who get slapped with a reckless driving charge, though if anyone is so stopped and caught, they are likely to be arraigned , a bond maybe set ( for just that charge, often a small one) and/or the case is decided right there with a fine, thrown out , or the person sits in jail. The law does give the judge that leeway to hold someone who may be dangerous that way.
Given what LE's suspicions were and are about JM, and also given his erratic and explosive temper, reactionary about things, the safest place for JM is probably jail for even for himself. Family, friends, loved ones and the on e in jail may take it hard, but really, it can be the best place for a person, even one innocent, until a cooling down period can be given, and the dust cleared as to what charges will really stick.
Sometimes this leeway is abused, yes. In this case, I don't think so. A upset JM could result in others getting hurt. That he's in custody reduces those chances. Also public sentiment is such that it isn't safe for JM to be out there roaming about.
So LE chose to take the latitude the law has, in this case.