Spectrum license fees are one way to encourage spectrum users to use spectrum efficiently. The trick is to set the fees properly; if they are set too high, then spectrum will be underutilized with respect to their social benefit, and set too low and it doesn't accomplish efficient utilization. On page 126 of his recent budget proposal, the President is proposing increasing revenues from spectrum licenses to US$550M to US$50M over four years, 1000% increase.
There is little but speculation about which spectrum will be taxed, since the Office of the President is apparently saying little about this. The PFF has remarked that, if applied to mobile carriers (which have already paid for licenses through auctions), the tax is quite regressive. According to this item, this fee may just apply to broadcasters. If this is the case, it will help motivate broadcasters to refocus on being programming producers (and possibly aggregators) since it will cost them ever more to transmit their product to an ever-diminishing audience at even higher costs than before. Michael Marcus just posted this related item.