The person who got charged $8 for the taxi "parking" likely got ripped off.
I can't find the details right now, but Disney charges a yearly fee to taxi drivers to let them operate without paying the daily parking fee. They also offer a monthly alternative, I believe. In any case, the taxis only have to pay once per day anyway - just like anyone else paying to park (you just show your reciept).
In the case mentioned above, where her DH was coming from an outside conference, it may well be the case that the taxi company doesn't regularly go to WDW, but rest assured you will not be charged the $8 by reputable "yellow cabs" in WDW. If you do, report it.
For more info on taking taxis at WDW, check this link at mousesavers :
http://www.mousesavers.com/taxi.html
I've only taken a taxi at WDW once or twice. I always think about it, but it's usually easier to just take WDW transport, unless you are going resort to resort. On my last trip I did want to grab one after
MNSSHP's a couple of nights, but at that point it was easier to wait for a bus on a bench that would bring be straight to Pop instead of having to get to the TTC first and then transfering to a taxi. Plus, MK and Pop are about as far away from each other as you can get, and I'm sure it would have been pricey.
N.E.D.