They were the proximate cause but not the ultimate cause. When the state is that dry, something is going to start a fire sooner or later, and when it does, it's going to be bad.
They are basically being penalized for being the straw that broke the camel's back.
Probably better maintenance would have helped some, but it's not like they bear all the blame for the destruction caused by the fire.
I would accept that argument if it happened once, but alas it didn't.
While anyone can start a wildfire, they are a large actor, which causes them. They make profit and could invest money to reduce their part in it, but as long that is an external cost, there is no economic motivation. That's not penalisation, it's economics.
I think implicit in PG&E's decision to neglect maintenance was that they weren't allowed to raise rates and it wouldn't actually be very economical to do maintenance to reduce wildfire risks, so shareholders preferred profits were returned to them rather than reinvestment into e.g. maintenance. So adding the "motivation" to reduce wildfires might've made them more likely.
I don't think anyone is claiming they bear "all" the blame. That would be an issue for a court to decide if it ever came to it.
Instead, the counterposition here seems to be that they should bear none of the blame, that any liability on their part is an injustice, and that refusing preventative maintenance and literally unplugging regions at risk is somehow an acceptable outcome. And that seems insane.
If they can't provide the service they're contracted to provide, they need to be regulated to be sure they do. The market has failed (and this isn't the first time PG&E got caught playing ridiculous games with regulators at the expense of customers, I was in California during the electricity crisis of 2000).
They are basically being penalized for being the straw that broke the camel's back.
Probably better maintenance would have helped some, but it's not like they bear all the blame for the destruction caused by the fire.