If a person were to park their car and then park hop by means of busses, they woulld be wasting their time. When you park in the park parking lots, you save your reciet and use it to gain free parking at any of the other parks that day. And when you travel using your own car, you will probably get their at about the same time because you wouldn't have to wait for a bus. I don't mean to critisize, but your logic does not make all that much sense.
First, you're assuming that the average guest would know that they don't have to pay for parking again if they get a receipt at the first park they visit.
Second, it is possible a family may not want to waste gas traveling from park to park, and also may just not want to have to leave their Disney experience once their in a park and have to deal with traffic. Therefore, they would leave their car in the parking lot of the original park and use Disney transportation to park hop. If my log is all that flawed, then why do so many guests do this?