Not something I personally have an issue with but it seems any condition that can be clearly seen (missing limb, use of wheelchair etc) is pretty much accepted as is, while conditions that can't be clearly seen seem to trigger the worst assumptions. The people who struggle with exhaustion, perhaps due to a debilitating condition like MS (that also can't be seen) or as an effect of something like cancer treatment, who are trying hard to live as normally as possible and not use things like crutches or a wheelchair, but who need to park in a disabled bay because they really do struggle to walk to the far ends of the parking lot, seem to be the ones assumed to be trying it on.
It also seems that mental health conditions sit somewhere between being accepted and being assumed to be little more than a whinge. We wouldn't tell a quadriplegic to stop fussing and get out of their wheelchair and get on with life, but somehow the person struggling with clinical depression is expected to just cheer up and get on with life.