The fact that they don't implies that they perceive some value is provided by publishing in the traditional way.
Assuming that journals die as a result of your law (why would people pay for a journal anymore?) is it worth considering that value (perceived or real) before advocating it is gone?
For example, consider a news article which starts with "a study published in Nature shows that..." versus "a study posted on Joe's blog on the internet indicates..."
For better or worse (and often with mistakes) journals act as a filter. Researchers in a field can "keep up". They can spend their time on "good papers" rather than trying to sift the dross of the internet.
Researchers, and by extension the place they work, gain karma points when publishing in hard-to-publish papers. If other papers cite their work they get even more points. Ultimately academia (employers and peers) ranks people using these points.
Does this mean your idea is bad? Not necessarily. But it's worth understanding that your proposal would "fix" one problem, but introduce a bunch of new ones.
Another (equally flawed) approach to solving the problem is that the govt simply pays for journal subscriptions to anyone who asks. This means "open access", with benefits limited to "their" population. But the values offered by journals are preserved. Of course this incentivizes journals to increase fees.