I recently visited a few airports, although most of them had standard wifi which allowed access after a landing page, I have noticed that some free wifi providers (Shanghai Pudong, Singapore Changi) have began to request for a random password to be sent to your mobile phone via SMS, after which your device will be allowed access. I was not charged by my mobile phone provider for either the SMS or the access.
Is there a meaningful rationale for why they are doing this? After all, sending long-distance international SMS certainly has a significant non-zero cost, and I don't see any obvious benefit in them doing this.