An unpleasant fact of law-school faculty life is that, at least at my school, I'm required to grade students so that the average is between 3.2 (a high B) and 3.4 (a low B-plus). Because of the nature of my course [0], a timed final exam is about the only realistic way to spread out The Curve.
Like it or not, there are life changing impacts to others by cheating at this stuff. This is unambiguously cheating.
I'm not aware of many jobs where employers don't care how fast the work gets done.
It really depends on the perception of whether the goal of the extension is to give disabled students an edge over "normal" students or to give everyone a fair(not necessarily equal) opportunity to complete the test.
To explain in more detail. The ADA says that an accommodation is when an entity (business, employer, school) makes a change of behavior. Installing a wheelchair ramp in an older inaccessible building is an accomodation. Granting extra time is an accomodation. Simply having accessible buildings or excessive time is not an accomodation.
But why the lawyers treat it differently. Business feel comfortable, when they have a ramp, arguing that no accommodation is necessary for the wheelchair bound. The standards of accessible physical design are clear. Schools do not feel comfortable saying that no accommodation is necessary for mental health issues, ever. Their lawyers advise them that it's much better to give some sort of accomodation and argue in court about sufficient accomodations vs giving no accomodation at all.
If you lie about having a disability to get extra time, you're falsely gaining an advantage over other students taking the same test. That's cheating.