But yes, of course this is not just about filtering out fake clicks. The draft proposal lists a bunch of use cases, most of which have nothing to do with ads.
You're right that attribution and measuring ROI is way harder and less precise for ads sold by impression than by click. That's why they're not the common form of advertising, especially on these kinds of ad networks. But for cases where the ads are per impression, the concerns about fraud would be exactly the same. It's not about a crawler accidentally generating impressions, it's about bots deliberately doing so.