Is he who participates in a campaign to lower internet prices sinful if users watch forbidden things or commit other sins via the reduced-price internet?

There is no harm in participating in the aforementioned campaign, and this is not considered prohibited assistance. This is because the assistance – if it exists – is neither direct nor intended. If this were considered prohibited assistance, it would lead to the prohibition of many permissible things. It is advisable to disregard whispers (waswas) and overthinking.