This is just a guess.....but I suspect that the gift cards are donated by Amazon in some way, shape or form. If that's the case, then there's no cost to WGT for the prizes.
Giving an equivalent value of credits to ineligible winners would "cost" WGT through lost revenues.
Like I said, that's just a guess.....but it seems like the most probable and logical reason.
Also, I fully agree that Fabian (and anyone else in the same situation), should be allowed to play in the April final, whether or not he's eligible for the grand prize. After all, what does it hurt to include him? He's certainly earned it.....