I think it's a bit of a gray area because of the government involvement in running most airports.
I certainly don't think would price fixing for a stadium owner, for example, to set maximum price levels for concessions at the stadium. The owner has a vested interest in getting people out to events, and since one way to do that might be to offer reasonably-priced food and drinks, they should be able to tell their food service contractors what they can charge. (Not that any staidums have tried this approach . . .
)
Similarly, airports (may) have an interest in making travelers' experience more pleasant to encourage more connecting traffic or maybe just to give travelers a better first impression of the city/region. Reasonably-priced food (and free WiFi, as another poster mentioned) could certainly be part of this effort. In that sense, airports should have the right to dictate terms to their vendors to be able to provide the overall experience they want. It's still a market system -- if they make the terms to onerous, they won't have much choice in vendors, so the airport authority can't be too harsh in their conditions.
Where it gets gray (IMO) is that airports are frequently run by a government or quasi-government agency, and in most cases, they don't have any real competition. Even where there are multiple airports near each other, they are often under the same authority (e.g. BWI-IAD-DCA). Business that want to provide services for airport travelers don't have that many options, so I'm a bit leery of giving the airport authorities
too much leeway in dictating terms.
That said, in the end, I think that setting price caps for airport food is ok. I think the interest of the airport in providing a better overall experience for travelers trumps the desire of a business to be able to charge whatever they can to a somewhat captive audience. And the business definitely know what they're getting into, so they can determine whether it makes business sense for them to have a location at the airport.