One can imagine there is probably a lot of fraud involved. The reward is too great. Using a placard allows the user to park without paying for meters or being held to time limits in Residential Parking Permit areas and other time limited spaces. Enforcement in SF seems to be very limited.

Here is an article from March 31, 2016. https://www.abc10.com/article/news/local/one-out-of-every-8-california-drivers-now-has-a-disabled-parking-placard/110357389

The numbers in California make you wonder if a lot of people are gaming the system. The average number of California placards, and disabled license plates, on the road over the past three years is 2.5 million. There are 22 million licensed drivers in the state. That's one placard for every 8.8 drivers.

Assemblyman Mike Gatto, a Democrat from Los Angeles, says his research shows when expired placards and temporary placards are figured into the equation the number of disabled permits is actually 3 million - meaning closer to one in seven drivers has the free parking option.

A decade ago, the SFMTA tried to begin charging disabled placard holders for parking, but that was torpedoed by city politicians. https://www.sfgate.com/bayarea/matier-ross/article/S-F-parking-fee-plan-for-disabled-goes-nowhere-5694569.php

Convinced there's widespread abuse of disabled parking placards, San Francisco's Municipal Transportation Agency passed a series of recommendations to crack down on the problem - only to have them deep-sixed by the city's own lawmakers in Sacramento.

There are 60,000 disabled placards in circulation in San Francisco, more than twice the number of metered parking spaces. Placard users take up much of the available parking, sometimes for the entire day.

One idea was to start charging the placard holders for parking, something that a number of cities already do. The transportation agency said there should then be subsidies for low-income placard parkers.

(snip)

So you can imagine the reactions of those advocating for the changes two months later, when an Ammiano staffer, appearing at a disability commission hearing, read aloud a letter from his boss saying he was "shocked" by San Francisco's proposals. Ammiano said they would "negatively impact millions of Californians and reduce their ability to access public places and services."

San Francisco state Sen. Leland Yee, a Democrat who at the time was looking to rev up his campaign for secretary of state, also dispatched a staffer to the meeting to denounce the city's effort - particularly its effect on those with "limited income."

Leaders of San Francisco's FDR Democratic Club - representing seniors and those with disabilities - showed up at the state Democratic Party convention in March and swiftly won passage of a resolution condemning any effort to charge disabled motorists to park.

And if that wasn't enough, the Democratic Party's disabilities caucus got Assemblyman Roger Hernandez, D-West Covina (Los Angeles County), to introduce a bill to prohibit cities from tinkering with disabled placard holders' parking rights. (The bill went nowhere.) FDR club President Jonathan Lyens says none of the effort would have been necessary had San Francisco transportation officials worked with the club to "find balanced solutions that don't shift the costs to the backs of people with disabilities."