There are some legal issues here that, I suspect, will eventually have to be settled in court because there are contraditions.

If somebody says "you do not have permission to touch me there" and they are touched there and it causes pain (i.e., "injury"), that satisfies all the elements of a crime (called either "assault" or "battery" depending on the jurisdiction). There are similar issues (discussed in another thread) about sexual touching.

On the other hand, the courts have held (and, rightly so, in my opinion) that once a search is started, the person cannot withdraw from the search.

Those two very accepted legal principles are in conflict in situations where a claim is made (by TSA) that a continuation of the seach requires the granting of one of the permissions above.

Then there's the American's with Disabilities Act. TSOs have claimed here that "if we cannot clear a person, they will not be allowed into the sterile area". Making that statement in a general way is a prima facia violation of the ADA. If the TSA doesn't have the available techology to clear a disabled passenger while dealing with their disabilities, that's the TSA's problem: it doesn't somehow become the passenger's problem. The law does not allow a disabled person to be denied access to the sterile area simply because the TSA doesn't have an effective way of screening them.

Now it's certainly true that the above is no more than legal theory since it's never (to my knowlege) been tested in court. However, I strongly suspect that the DHS legal office wouldn't want to have such a test case because of the real risk they'd lose.