Economics are one of the last things that are discussed.
From what I have been told, the Company tells the Union how much they are willing to spend on their labor costs. The Company and Union then decide how they are going to split that money between FTers and PTers.
Both see the need to address PT wages, whether it be due to pressure from other employers or by individual states raising their minimum wages to upward of $15/hr. The fact that the Company felt pressured to use bonuses to supplement their starting PT wage just to make sure new employees showed up to work tells me that PT wages are one of the hot button issues.
FTers make more than enough money.
It would not surprise me to see the starting wage for new PT hires raised to the $13-15/hr level in the new contract.