For the sake of argument, if we assume that national governments are neutral in this, then they can place a proper value on that environmental cost. The downside of this, of course, is that means that the cost value will be set by politicians.
Even if the cost assigned is incorrect (too high or too low) at least it starts the conversation. Some energy generators do not like the benefits/subsidies given to other classes of power generation and some energy generators can point to their type as having a lower environmental cost. It is also obvious that some environmental costs are unknown as we simply everything about what we're doing.
What needs to be recognized that damming the river to generate power has an environmental cost, not just a construction cost. Wind turbines have downside. Solar panels have downsides. Nuclear fission (and potentially fusion) process have environmental costs, including the mining of the raw fuel. Fossil fuels have environmental costs. If we fail to take into account these costs (i.e. if we fail to recognize the downside to any power source) we risk basing our economy on a false values. If the governments let coal burners dig coal out of the ground for free (no royalties) but charge natural gas burners royalties, we can imagine that we will lean toward burning coal, as it may be cheaper due to the royalties.
Right now, the economics of long term damage are wrong. Individual companies are taking profits from risk they are downloading to the state. The state has no method for properly obtaining money to fix the problems created by the companies. If the cost to the company reflected the environmental cost for that product, then the market will move to the least cost options first.
you've asked "who". The short, sad answer is the same people who decided subsidies in the first place. Politicians. If they can hand bucks to folks to generate power, they can hand bills to folks who generate power in the same way. Hoping, of course, that their decision point of view is for sustainable economics, and not short term advantage.