You pay taxes on the income that you make when you sell your power "into" the grid, which makes sense as you're acting in a similar role as the utilities.

I think it comes from a more collectivist mindset. To an individualist, this seems pretty foreign. I think it goes something like "If you're able to do lots of stuff for yourself, you should have to do something for your less able neighbors as well". The danger is as pointed out, if the burden becomes too high or feels to intrusive, you'll get "well screw it then, I'll just be like everyone else".

Intentionally or not, taxes are a way of discouraging an activity. If you tax solar, then there is less economic reason to use it. At least until the benefits become higher than the taxes.