There can be a genuine need to make it fair. Some students with anxiety can take 10 minutes to read the first question, then are fine. ASD could mean slower uptake as they figure out the exam format.

So let's say you have a generally fair time bonus for mild (clinical) anxiety. The issue is that it's fair for the average mild anxiety, it's an advantage if a student has extremely mild anxiety.

As you say, hopefully the test is not overly time focused, but it's still an advantage, and a lot of these students / parents will go for every advantage they can.

If speed is important, it's important whether or not you have clinical anxiety.

If it is not important, why are we evaluating it at all?