Think about it: when you pay for power you pay not just for generation but for distribution as well. When you feed it back and your meter runs backwards the power company should be paying you only for generation (at a wholesale rate, so they can turn a profit).
In some countries they measure your power in both directions and give you a credit based how much you put back into the grid. This would be much lower than what you pay to buy power.
Why would be good to have tax money go where no private investor would? How sustainable is this in the long term? I thought this was obvious enough, apparently not.
Just to be clear, I'm not agreeing as I feel that the State investing in the private economy can lead to good things if done responsibly. The problem is often that the people doing the investing have no idea of what they are investing in, which can lead to economic damages. Thankfully in these cases it just appears that we lost taxpayer dollars in those failings.
Personally I feel that many of those firms, not all mind you, were simple pump-and-dump schemes that had no intentions of succeeding at anything.