I hate to say this because my favorite cruise lines all do it - but in most cases the phrases "free air" and "two for one pricing" are just come-ons - they have been saying it for so long it is even in their yearly brochures.
That obviously means the air is not "free" it is just included in the rice of the cruise. The reason they do this is because market research told them that people who see airfares higher than cruise fares will back off from buying the cruise simply for psychological reasons (not practical reasons). So they stopped showing the airfare as a separate charge and just added it in with the cruise fare.
In the long run you are going to compare the entire cost of the trip side by side anyway - and you will see which cruise (with air) is the best deal, whether it is "free air" or not.
Now - once in a GREAT while I will see certain cruise lines offering big discounts for cruises with air included - I have seen this with Holland America, Grand Circle, Celebrity and others - when it happens infrequently or only on certain itineraries I consider a real bargain, but when certain cruise lines (Oceania, Viking River, Regent) continually offer "free air and two for one pricing on EVERY cruise for years, you can be pretty sure that is not a special deal, just a fancy way to show their pricing.
I personally dislike it - but they report they are very successful using that strategy, so I cannot fault them for doing it.
What it says to me, however, is that travel agents are telling clients "this is a special deal; two for one pricing!" But it isn't a special deal in most cases, it is business as usual.