Actually they purchase the data normally (*544#) from Safaricom and resell in smaller quantities. See the price of 25GB and do the maths. This business started after Safaricom introduced Sambaza Internet.
As for the airtime some of those who sell the data receive payment in form of airtime, which now they have to convert to hard cash and the only way to do so is to sell the airtime but at discounted rates so as to attract customers.
Initially when the Sambaza data business started payment was in form of Mpesa but as competition increased some started accepting airtime, saving the customer the transaction fees.
Its more or less the same with Bonga Points. When Safaricom made it possible to share Bonga points someone tried to start a Bonga Point exchange which I think didn't take off. But then if for my 100 points you can give me Ksh.30 airtime instead of Safaricom's 20 then I send you my points and receive the credit..
What I was wondering is whether at the end of the day the 'rogue' data business will affect Safaricom revenues. I think it increases their data sales...
Know the market- Don't invest blindly.