Everyone is already infected with the virus anyway, (revealed at the CDC in season 1).
I see it as more of a 'fuck you' to the Saviors than a future plot point. And its smarter for the kingdom to feed them walkers and not use their own food supply to fatten up the pigs.
That's right. Everyone IS infected. I just have a hunch that eating that food will do something. If not, the fuck you is good too. Ha ha,
TIS