Solar is cutting into the margins that the plants need to be profitable. So the plants increase the base rates they charge so that they remain profitable. This increase is then paid for by everyone (not just the solar owners). But it's still a net win for the solar owners, because they're the ones saving the most when rates are highest (when it's hottest, and ACs are cranked up). So effectively, doing nothing (and letting plants increase rates) acts as a net wealth transfer to those with solar panels.
This isn't in the interest of the common good. So the utilities try to soften the costs by taxing the solar owners. But then the math for the solar owners is thrown off because they anticipated saving more/making more over the life time of the panel.
I don't know what the right answer is.