Yes to some degree I think working in a special school has desensitised you, and although you don't say your age by the tone of your post I suspect you are older than 17, with more general life experience as well as specific experience with as you put it "adult sized people who are nonverbal, struggle with impulse control and sensory input and have the emotional and intellectual ability of a three year old".
You went into that job reasonably knowing that you are going to be exposed to this, you have likely received training on how to deal with it and de-escalate it, and you have the experience of what works and what doesn't. The 17 year old girl in this scenario doesn't, and it's quite a reasonable expectation that someone working in a shop doesn't have the skills or experience to deal with the same things you do in the same way.
How can you expect that 17 year old to have the same experience in life, never mind specific experience with people with impulses they can't control, to have the same viewpoint on the incident as you?
That's as unrealistic as expecting disabled people to not be in public. You're saying that the OPs daughter should have empathy and understanding for the young man in this situation, because of his condition, however you're not willing to apply that empathy and understanding the other way around. You think she's 'allowed' to be upset, but no more than that, that's unreasonable. It's not for you or anyone else to dictate how this affects her, in the same way it's not for anyone else to unilaterally decide what this young man's intentions were and decide he needs to be locked away from society indefinitely.
I would likely be much calmer in a situation like this, I work in hospitality, so also have a reasonable expectation that I won't be abused and assaulted because I simply work with the public, however I'm in my mid 40's and have experience with people who have disabilities, that life and specific experience is going to affect how I view and deal with the situation, again a 17 yo, who is still a minor, still expected to be in education of some description, is not going to have the benefit of that experience.
Choosing to be working with the public should not be a reason to be assaulted and just live with it. No matter the reasons behind the assault. She wasn't in the wrong place at the wrong time, she wasn't doing anything wrong and quite frankly it's a lazy way to look at it and sweep away the fact that working in a shop shouldn't expose you to assault or abuse just because you work with the public. That shows a real lack of respect for anyone who does these jobs. Do you expect that anyone just outside in public should be accepting of an assault by virtue of just being in public?
You're showing the same lack of empathy and understanding that you're accusing others of, that you're so disgusted by, while demanding that empathy and understanding for those you care for and others like them is non negotiable.