Because I thought that argument has been hashed out elsewhere and I didn't have anything to really add. But okay.
Because I don't understand why they shouldn't be able to charge more for a higher level of service. In most cases, you're talking about businesses that are willing to pay a premium. That ability to charge more make it easier to generate more revenue from a given network, making investment in additional service/improvements more likely. Those who are willing to pay a premium often are what enable (financially) investments that benefit all of us.
So if Progressive Insurance is willing to pay a huge premium for faster service, they're making it more likely that the providers will spend more to improve those lines, which is something that likely will benefit the rest of us who aren't willing to pay those premiums. Conversely, if you have to charge everyone the same and provide the same level of service, they're not going to be able to milk extra revenue from those businesses. That makes the lines generate less overall revenue, which makes expansion/investment less likely.
It's just one of those populist issues, where equality is a higher value for some people than is quality of service. It's like the old Russian joke about the farmer who has only one cow, and watches with envy his slightly richer neighbor, who has two. He complains bitterly about this unfairness, until one day, his Fairy Godmother pops down, tells him she has seen his sadness and only having one cow, and offers to grant him a wish to put him on equal footing with his neighbor. He thanks her, thinks about it, then says with a smile "okay -- kill one of his cows".