>The safety reasons are real though: when linesmen need to do work on the grid, it has to be depowered. This used to be easy to accomplish: just make sure the line is switched off from at the substation and you can be reasonably certain that the line is safe to work on. If Joe Random decides to send his solar power directly into the depowered grid, that would be a Bad Time for the worker involved.
<yawn> This argument again? People have been saying the same thing about backup generators forever and it's just not true in practice.
From the POV of your solar or generator the resistance of the neighborhood is basically a short. Breakers will pop if generator and/or or voltage will collapse if solar. If you are out of sync and the grid comes back your hardware will be what leaves the chat, not the grid's
Linesmen are trained to ground their work first because the far bigger hazard to them is miscommunication among upstream coworkers. Better to liquify fuses or hardware than people when that happens.
If "everyone" had solar or a back fed generator it could be a problem but we'll cross that bridge when we get to it. And that bridge would be crossed procedurally on the utility/linesman side (grounding your work mostly already does but more would likely be desired). They're not stupid. They're not gonna trust their lives to the assumption that the hardware upstream and downstream is going to do what they expect.
>"Protectionist reasons" sounds nice to people who already hate big corporations, but it doesn't make a lot of sense. When the grid is down, it's down. There is no revenue to protect at that point. The actual thing they might want to protect against is big solar farms competing away revenue from fossil fuel plants, but as you'll notice such solar farms are entirely legal.
The protectionist angle is that if generation was more local rather than regional there would be much more competition because it would become more economically realistic for municipalities to strike out on their own and/or parter with each other and do so in order to shake off existing overpriced or incompetent utilities.