We have the same thing here. Probably since year 1 of 22.3. I have actually seen drivers swarming around their On Cars in the AM, all asking for a 22.3 'helper' for part of their day.
When you say majority, how many is that? Is it 76, or 101, or what? Have they all been put up for bid and filled, or 30 per year of contract, or what? We need some more facts.
Umm, because that's what was negotiated? Ask the negotiators? And ask the IBT, as I'm pretty sure they had to review & approve of it? After all, it's not like PT air walkers get PT air driver pay, or peak season helpers get pkg driver pay, or even peak season driver pay.
Let's see now, 22.3 top rate is now $28.14 right? $28.14×2080=$58,531.20, plus health & welfare and pension. Sounds like a pretty good deal.