As a service to the public, this is immensely helpful given the consistently rising prices of fuel products, but as a business model for even public goods, this is not sustainable. Public goods and services are only feasible if they are paid for by society at large. This can be done in the form of taxes, but also specific charges for services where use can be tracked. In the case of a public transit service, it is easy to track how much is being used and by whom.
The Metro Lines in Islamabad (or other cities for that matter) were designed to be low-cost options of travel for residents of each city, but waiving the cost entirely means that the government is essentially bearing all expenses for the service. This means that not only the fuel cost but any repairs or service needed on the buses themselves or the station will have to be borne entirely by the government.
Instead of allowing for a free ride, no questions asked, what the government can do is offer further discounts to the poorest users of the service. Offer subsidies, off-peak hour discounts, and free rides for those living below the poverty line, but let those that can pay for the service they utilise. This is important for the long-term viability of our nascent public transport system, one that still requires extensive development—not just in Islamabad but throughout the country.