Very few current broadcasters "got their spectrum for free." The FCC has been auctioning off broadcast spectrum for over a decade, and broadcast stations that were licensed before that time have typically been sold and resold at "fair market value" many times over the years. As a result, it is a rare broadcaster that currently holds a broadcast license obtained directly from the FCC "for free". Most broadcasters have paid dearly for that license, both in terms of the station purchase price and the public service obligations that come with the license.
Still, fee proponents argue that because the original license holder didn't have to pay the government for the spectrum, the "free" argument still applies, no matter how many times the station has changed hands since then. That argument is eviscerated, however, by a simple analogy. When the United States was settled, the government issued land grants to settlers who "staked a claim" to virgin territory by promising to make productive use of that land (the "Sooners" being one of the better-known examples). Other than the promise to use the land, these settlers did not pay the government for their land grants. The land then passed from generation to generation and from seller to buyer many times in the years since the original grant. However, despite the fact that the original owners "got their land for free", I would wager there are few homeowners among us who would agree that we received "our" land for free, much less accept a governmental fee premised on that assertion.