I actually think it is a lousy argument, but a favorite one for utilities, who receive power with no capital investment in their part.
An (the?) underlying problem is that the cost of the grid (poles, transformers, wires, and maintenance thereof is currently billed on a per kWh basis, when it really is a per meter per month cost. The trouble is that if you do that two things happen; a) power ends up costing very little reducing consumers' incentive to conserve, and b) lower income folks are hit with really high monthly fees before they have used any power at all. I ran the numbers for my local area, northern California, and the per month fees end up in the several hundred dollars/mo as a meter fee. The existing, hidden, subsidy of large homeowners with large power bills carries along many, many lower income users. That makes a per meter fee politically a hot potato, and the utilities don't want to be the evil overlords socking it to the poor, so they are starting by socking it to the evil rich bastards who put in solar as being "free riders".
It is being thrashed out here as we speak. I suspect that this is only round one of many until we edge over to a per meter fee, with subsidies for low income consumers. Right now, the proposed tariffs incentivize pulling the plug on the grid, adding solar, and batteries and moving on with life, which will really hurt low income consumers when they have to pay for the whole grid. To say that it is complicated is an understatement in my opinion.
All the best,
Peter