They did so on the basis that they are common carriers, and do not have speech rights such that the net neutrality would interfere with.
The Wall Street Journal reported that:
In the core decision, the D.C. circuit court ruled the FCC had sufficient basis to impose utility-style regulation on broadband service, because consumers no longer look to internet-service providers to provide the online content they are seeking.
The appeals court’s 2-1 ruling sided with the FCC in a dense, methodical opinion that rejected the challengers’ various arguments one by one.
The Court was said by the New York Times to have decided to treat broadband as a utility, not a luxury.
The actual judgment is found here.
The absence of any First Amendment concern in the context of common carriers rests on the understanding that such entities, insofar as they are subject to equal access mandates, merely facilitate the transmission of the speech of others rather than engage in speech in their own right. (at page 110)