In a totally free market, yes. But as a utility provider ISPs should not be considered a free market. In exchange for near monopoly power over the internet access of a particular region (in the US), these companies have to play by extra rules protecting consumers’ access to 3rd party content.

Imagine if your electric company charged different rates because of the brand of electric car you charged. Sure, they can charge more per kWh based on time of day, total household usage, or total network availability of power at the moment, but charging more (or less) per kWh going into a Tesla than a Leaf would give that electric company monopoly too much power to pick winners and losers.

I like to think of ISP deep packet inspection using the same analogy; I for one would not like my power company to know the make and model of everything I have plugged in in my house, beyond total usage it’s really none of their business.

I recognize these aren’t perfect analogies but hope that they help get the point across.