The panel most likely was pulled from a working game, but back in 1985!
The true answer to this one has little to do with people being boneheads, and probably has much more to do with the fact that there are probably several gamesellers out there that have people waiting to pay $800 - $1200 for a Tron, and they need the panel to finish their machine off.
Sure, they could wait until later, and add another $100 to their profit margin, or they could buy now, sell now, and then immediately reinvest that money.
This seems to happen from time to time with assorted pieces to different popular games. You ever seen what Assault* parts sell for? It is just as bad as Tron, even worse really, considering that complete working Trons are worth 2 or 3 times what Assaults are.
* Assault parts go crazy high in prices because it is a semi-common game, that can't be converted. It is too good to simply part out, and thus everyone tries to repair them. Thus you have the situation where a good working Assault will sell for $300, but will part out for like $500+.