https://images.apple.com/environment/pdf/Apple_Environmental...
As opposed to what? trashing it? You'd rather throw your iPhone in the trash than just hand it to them when you're in the store already?
It's the throwing it away that is the problem.
Not having a phone in the first place is the best for the environment. Failing that, having someone else reuse that phone is best. Only if all else fails is recycling the preferred option.
So of course people are going to concentrate on the problem of people just throwing these things away. And that's for anything. Not just phones.
Though the cost of responsibly recycle and dispose of an old computer might make the $0 offer actually a decent one.
Google do similar, as do most electronics retailers.
Is that not not a thing in the US? Perhaps it ought to be.
If they offer even anything, you'll get a lot more pickup - everyone will learn "get a discount at the Apple Store if you bring in an old PC" and reduce the amount of electronic waste.
However, done too well or for too much, and you could greatly reduce the availability of older still-working machines.
I expect most of that 30% recycled material is from other sources than traded in devices.