That may be true, but from an engineering perspective it's unwise to build a two-story house on a sand foundation just because the rest of the neighborhood has done so.
FWIW, telco companies in Mexico don't charge for WhatsApp data transfer. As for prices, a 30 day Telcel 4.5GB plan is 200 pesos ($10) per month. So long as you don't use up the data doing things outside of WhatsApp, like surfing the web, which will expire the plan, then you get free WhatsApp (and Instagram and most social media) transfer for the month.
So it's not the worst organic solution and it's not even obvious how to centralize a better solution. For example, how many subway drivers are there and how much govt bureaucracy would be needed to do something like give everyone a $10 stipend for WhatsApp? Even giving each driver an extra $10 pesos per month wouldn't change anything.