> Because you'd typically have to collude to do it
There's all sorts of ways that companies charge the wealthy more for the same product without collusion.
1. Coupons
2. Generic brands that are repackaged name brands
> anyone being charged more would switch to the other company
And this is a common way to charge the wealthy more for the same product. Someone making $500K+ is less likely switch ISPs every year, or even spend the time to threaten their ISP they are going to switch.
That's doing it by proxy, though, and isn't what I took OP's question to mean. S/he specifically mentioned a comparison to income tax, which is much more direct.
There's all sorts of ways that companies charge the wealthy more for the same product without collusion.
1. Coupons
2. Generic brands that are repackaged name brands
> anyone being charged more would switch to the other company
And this is a common way to charge the wealthy more for the same product. Someone making $500K+ is less likely switch ISPs every year, or even spend the time to threaten their ISP they are going to switch.