They shouldn't. Two other positions exist which would be more to libertarian liking.
One option would be to oppose the sanctioned monopoly that net neutrality supporters want to regulate. By providing competition, it will be easy for the free market to punish ISPs who choose to discriminate between traffic sources. A side effect of this choice is a snarl of wires on the telephone poles - one for each local ISP. Also, this option is unlikely to work where all communications lines have been buried.
Libertarians could compromise to a second option - the common carrier option. This position yields the physical connection to stringent government control (like telephone networks, power grid, etc) but allows any company to make use of said infrastructure. Essentially, the wire is socialized, but everything that it carries is free-market.