For most utilities, a customer that goes from a $300/mo bill to $10/mo will trip a review flag in the system. You might be surprised then to know that a utility tariff is a legal contract between the customer and the utility. Violating that contract can have legal consequences. Good luck when your customer gets in trouble with the utility and pulls the contractor into the scuffle for aiding and abetting. What is most likely to happen is the customer is assessed for any lost revenue to the utility, fined for the violation, and the PV system locked out until it is permitted. Maybe the contractor is sued by the customer to recover that and the contractor is reported to the state board for a license review. Is a guerilla PV system really worth all that?
I'm skeptical. I'm skeptical of your first sentence because I've seen enough cases where it didn't go down like that, say when a customer had an 18th remodel but didn't replace the service panel. I suppose there might be flags in the sytem, but nothing seems to happen. As far as the rest...
First of all, one way around breaking any law or agreement might just be to apply to interconnect as a non-exporting system, thus avoiding any fees associated with net metering. So there's that, which just sort of takes the bite out of all of this.
On the larger point though, while I'm not skeptical that there would be some battles along the lines you describe, I doubt that the utilities can really push policies that incentivize a black market in electricity and then also marshall the extra resources to punish those who participate it. That doesn't work for most markets. I'm also doubtful that the typical service agreement obligates the customer to use a certain amount of electricity along the lines you imply. If I just use less electricity, then I just use less electricity, same as if I went on vacation or my air conditioner broke and I couldn't afford to replace it. There's no claim of 'lost revenue' for electricity that they didn't provide. A good lawyer will be able to fight back against any claims of harm against the utility as frivolous. And in the many cases where the bill might go from $50 to $10 or $300 to $200 instead of what you suggest, there will be less lost revenue to pursue per the cost to do so for each customer.
What it will come down to in the end is the money utilities can spend on lawyers vs. the bad press they'll get and the consequent erosion of public support for the regulations that might allow them to win those cases.