The reasoning I understand is that it enables the sort of monopolistic practices that historically have been restricted. Offering $20/mo "Facebook only" or $10/mo "Just email for Grandma" plans has providers competing more on what destinations they (don't) restrict in their Internet access, and less on actual speed/latency of the service. Larger providers could then roll out near-$0/mo "Email-only" plans at below cost, specifically to squeeze out the smaller providers previously trying to eek by on their equivalent $10 plans, and then retire those plans once competition is gone. Selling at below cost to kill competition is predatory pricing, which has spawned antitrust claims in the past in other markets.