Since its not illegal to park on them! there are myriad good reasons for parking on them within the law!
Given that some consider parking fines legitimate business costs
Is a pretty pointless statement, as for the vast, vast majority of businesses parking on double yellows is permissible (loading and unloading of goods irrespective of size as per Jane Packer flowers adjudication.)
white van man unloading his wares. He was on double yellows, blocking traffic and there was a warden writing out a ticket. He didn't appear to care one way or the other.
Unsurprising really – ticket was almost certainly invalid and would get cancelled – bloke handing out ticket isnt bothered, he hits his tickets target for the day (if he actually understands the law that is!)