I get it - street cleaning are "easy" tickets to write in bulk, and therefore efficient ROI for PCO time, but they're not the most important violations to cite compared to safety-critical things like blocked bike lanes (which SFMTA has an official policy to completely ignore citizen reports thereof), double-parking, or red zone (including daylighting) violations.
Part of the issue is improper fine structure (though I think this is at least partly controlled by the state?) - tickets for blocking a bike lane are rarely written and therefore it's a good bet to just do it and odds are in aggregate it's cheaper than paying for parking legally.
UPS, FedEx, Amazon, Uber etc rely on this as a cheap cost of doing business, externalizing their costs onto the safety of the public. SFMTA even offers bulk payment discounts to UPS, when they should be charging escalating fines for repeat offenders.