I think that's not an unreasonable position on the surface.
But I think the rationale is people typically protected by these laws are in positions where they are compelled to interact with people at higher risk of being violent. For example, an emergency room RN can't easily refuse care for a patient who was shot and bleeding, even if the person may give the RN concern they are likely to be violent (yelling angrily, heavily tattooed, clenched fists, etc). Similarly, a police officer can't easily refuse to arrest someone who has broken a law just because the person is angry and may be giving similar signals of potential violence. I think in these cases the law serves to raise the consequences of violence and hopefully decrease the likelihood violence will occur in the first place.
In the case of mental illness the water gets murky because we interact with patients who are violent for different reasons. In my experience, the most commonly violent patients are intoxicated and have cluster B personality disorders...in other words, people who could rationally choose not to be violent if they wanted. An example here is a patient who wants Valium and attacks an RN when he/she doesn't get it. This is clear acting out related to the person not getting what they want. We also have truly psychotic patients who become violent from delusional motivations...for example, thinking the hospital is a space ship and the staff are aliens planning to kill them. I don't think ethically we can hold the second patient nearly as responsible as the first. Ultimately the district attorney should have broad discretion to pursue charges or not...and it would make sense for the victim to be able to weigh in on the issue at a later date once the emotions have calmed down.
I think the motivation behind the proposed law change is for patients like the second example. But if the law was changed, every antisocial patient who attacked a nurse would be able to claim they were mentally ill. Beyond this it potentially opens up a pandoras box because any patient who attacked hospital staff could claim mental illness and it becomes difficult to prove the patient wasn't experiencing some diagnosable condition, even an adjustment disorder, because who in the hospital isn't experiencing some distress. This could broadly negate the previous laws in place to minimize hospital violence in the first place.