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The Oxford Advanced Learner's Dictionary gives the following definition of airtime:
the amount of time that is paid for when you are using a cell phone
And it goes on to give the following example:
This deal gives you 180 minutes free airtime a month.
I'd like to know why "free" is used at all. Does "free airtime" refer to time that is offered to the phone user as a gift? If not, why is it used at all? (By the way, shouldn't "of" have been there to connect "180 minutes" to "free airtime"?)
Far be it from me to argue with any Oxford dictionary, but I'm surprised at the definition your dictionary gives - unless it lists the other possibilities referred to in the sources below.
Clearly what the dictionary is talking about is the amount of time that you can use before you accrue additional charges. And it appears to be using free airtime in the sense that any number of companies now offer people free gifts. Unless the contract offers you a bonus of additional airtime that you haven't paid for (or won't pay for) at all, it's hardly free.
Why the expression is used that way is presumably because it's widely understood in that sense. But I agree that you have good reason to query it.
airtime is time spent "on the air", whether that be on a TV network's broadcast spectrum or on a phone network's spectrum.
There's nothing in the meaning that has anything to do with money.
That definition is akin to defining food as "the sustenance you pay for at the supermarket". That definition might make sense for groceries.
free airtime is like free food. The phrase is meaningful in contexts where you would normally have to pay. A separate definition "the amount of time that is paid for when you are using a cell phone" is not required by the fact that many phone plans allot customers a limited amount of airtime for the money they pay to the phone company, or by the fact that you have to buy food at the supermarket. Sometimes I think that advanced learner's dictionary must have been put together by advanced learners. In any case, learner's dictionaries are less interested in lexicography than in being a Baedeker for "visitors" to the language.
With respect to "30 minutes free airtime" it is quite common to have units of measure like "30 minutes" or "10 days" quantify a thing without partitive of.
He needs ten days rest after such an illness.
