I agree that it is not Tesla's responsibility to police whether a customer is following the rules of the ITC. That said, I think there are probably a few reasons why they have chosen to be very careful. One is perception - they don't want to be associated with tax fraud, as it could hurt their image as well as potentially sour Congress on future incentives (noting that incentives have been an important part of Tesla's business model starting out both with cars and solar.) Having an easy way for customers to grid charge could be perceived as tacitly enabling fraud. And, particularly in the solar business, less reputable companies have tried to abuse the credits (including fake safe harbor scams which don't apply to residential, and over-billing or including non-qualifying costs.)
Related, I don't think Tesla wants to be involved in a hypothetical IRS audit. Should they have a way for customers to enable grid charging, the IRS would potentially ask (demand?) that Tesla turn over records of who was using that feature since, realistically, the IRS is unlikely to go after many individuals for possible violations of the charging rule. But, if they could uncover hundreds or thousands of possible cases through one central place, that might change.
And, there is still the hassle of utilities. Tesla needs to maintain good relationships to ensure there are not issues that hold up interconnects. While it sounds like PGE may not have a hard rule, some utilities do, and sometimes things are more complicated (such as the rules being dependent on the rate plan.) It would certainly help if there were standards all utilities had for documenting rules and rates as well as for managing things like PTO requests. But that does not yet seem to be the case, and it leads to things like Tesla having bad information on a utility's rules.
I kind of think the utilities are the biggest issue right now. Conceptually, where they can verify the utility allows it, I think the ITC could be handled with a one-page document giving the customer two options - "I am claiming the ITC for my PWs and understand that as a result, Tesla will lock my system from charging from the grid (except StormWatch) for the next five years" and "I swear that I will not claim the ITC for my PWs, and as a result, I will have the option to charge PWs from the grid, subject to the rules of my utility."