That would make your ISP the de facto government for your online activities. A government that is potentially very invasive, inspecting your every data package, limiting your access to certain sites and dictating what kind of content you can download at full speed.
How is that libertarian?
A libertarian would say, people should be free to make and enforce promises among themselves, and the government should constrain itself to those roles that can't be performed by markets.
Libertarians by and large don't believe in "antitrust". The market is, a libertarian would reason, inevitably more powerful than any temporarily dominant firm. If a cartel of ISPs emerged from a free-market approach to Internet governance, that cartel would either (a) be an optimal solution to the requirements of Internet users or (b) eventually worked around and eventually replaced by alternate offerings in the market.