[Bartweiss]

Before it was stripped from the ACA, the price for not having insurance scaled with income. At the lowest end it was irrelevant because of Medicaid eligibility; above that it gradually increased from below-healthcare-price to above. So some people could still save money by opting out, but it pushed them towards the market by making the effective cost of insurance smaller.

(Technically speaking, it was a tax rather than a fine. That mostly mattered for legal reasons, but it did also work that way in practice. It applied to everyone, so insured people had the obligation waived, but the people paying it weren't in violation of any law or regulation.)