It might be a stretch of what was intended by the commerce clause, so I can understand arguing against the federal government's constitutional authority to regulate it. However, if the federal government lacks the authority, I would argue that the constitution should be amended to grant them that authority because the spectrum does need to be regulated.
As for why it is worth $6 billion dollars, it's worth that much because companies are willing to bid that much for it. The government isn't setting the price, they are allowing the market to set the price, which is as it should be.
Our government sets aside some frequencies for public use such as amateur radio. However, the amount of frequencies are limited, and the demand for them far surpasses the supply.
So how would you have our government allocate those frequencies? Should the government stem in and have a bunch of politicians that pander to special interests allocate them? Or should they place limited restrictions on their use and then auction off the frequencies and let the market determine how they would best be used?
Our government does a bit of both. Some frequencies are allocated by the government. A larger portion are auctioned off after some restrictions are placed on their use.
In similar circumstances, homesteading, dear tags, etc, they use a lottery. They have in the past used a lottery for spectrum/frequency allocations. . I admit I don't know if those were permanent or for fixed periods.
According to the link above, they've only been auctioning the spectrum for the last 5 years, before that they used lotteries, and before that they actually attempted to figure out what would be in public interest.
I guess I wouldn't mind them auctioning licenses for fixed periods. But I would like them to keep more spectrum for their own use, specifically for military use. The military would/will have to lease those frequencies from whoever buys them, or (more likely in the current budget climate) do without. In fact I know of military applications for which are having buy new equipment, because they can no longer use the spectrum the older equipment can operate in. It's geographic problem, as in some places they can use the frequencies, they just can't use them in other places they need to do so.