The abuse is not just with parking spots. Recently Disney World had to change their policy on handicapped access. In the past, guests at the park that were handicapped, were allowed to go ahead of the regular line and many rides had a special entrance to the rides just for thie. The entire family/group with the handicapped person were allowed to go with them ahead. What they found out was some people that were able to get a handicap letter from some doctor, were running guiding businesses for profit and advertising that if you hire them, they could get you "cuts" to the head of the line. So word spread quickly about this and many other guides started running the same scam. Disney is no longer allowing the handicapped guides to bring in people that have hired them but only family members get to go ahead with the qualified person.
The professional guides have to have a special badge to wear when guiding in the parks, so the combination of the special badge and a handicapped tag/letter makes it easier for Disney to spot.