Technically most plans provide additional data at a reduced speeds once the 3G data limit is reached. I'm assuming you're either referring to increasing the addition free data to 3G, or including free slow data with the $15 plan, if that plan doesn't already have free slow data.
The decision comes down to a business stand point. Other telecoms are charging high fees, and high overage fees. If their users go over their data limit, users may consider switching providers or request overage fees be waived. Anticipating a lot of users exceeding their data limits, those business took a proactive approach in waiving their overage fees. By waiving their overage fees less calls would be made to their company, which lowers operating costs, and less users defaulting on their accounts. These are both outcomes that are good for the company.
Here at PM plans are affordable so, I don't see users being burdened by overage fees. However, there is a case that plans that do not have data should be given free promotional data at slow speeds. The benefits of providing slow promotional data for a limited time would be: getting users on lower priced plans to see the benefits of upgrading to a plan with data, which would give them incentive to upgrade their plan at a later date; and allowing users to access information related to the COVID-19 pandemic to take appropriate measures, such as social distancing. I believe from an equity perspective, all plans should at least have access to data at slow speeds. Especially since the most vulnerable, who may not be able to afford higher priced phone plans, are the ones who most need access to information provided online.
