Who does the telcos' "differentiating" their internet services help, other than the shareholders of the telcos? That bit of rent seeking behavior would have large negative impacts on everyone else.
Why should we let telcos begin charging for no commensurate improvement in their end of the bargain after they've been granted local monopolies across the country? Internet service looks like a utility, quacks like a utility, it should be treated like a utility.
This is one of the two key dynamics in the tech industry. The other is distributors versus content companies. When Amazon streams you a movie made by NBC/Universal over pipes built by Verizon, Amazon is positioned antagonistically to these other companies in wanting to extract the largest share of the total you're willing to pay for the whole package. Net neutrality helps them do this, and so does weaker copyright protections, among other things.