"The state previously denied the application, saying the [Illinois] Roadside Memorial Act specifies that only relatives who lost loved ones in highway crashes may request memorials."
So, it's not that the dead were not human. It's that nonhuman animals have relatives who are incapable of requesting a memorial. PETA says "the cows suffered and are 'worthy of remembering.'" But Illinois can't be accused of discriminating against nonhuman animals, which seems to be the issue PETA is pushing. Roadside memorials are for relatives who request them, and no relatives of the dead have applied. And let's be sensible, even assuming cattle remember their dead relatives, symbolic displays don't jog their memories.