If they are actually parking, and there is an option to park "legally" further away and just walk, then YANBU. Generally this is the case so as a general rule people should not do it.
However, where I live, when you get "carded" by the post office you have to drive to this industrial estate which is in the middle of nowhere sort of but deep within this industrial estate is the sorting office. There are 2 parking spaces, one is disabled, so really only one if you are not a blue badge holder, and on the road outside these 2 parking spaces are double yellow lines, then all around the whole area are double yellow lines.
The only times I've turned up and the one parking space is "free" but also accesible, I have been blocked in by people parking on the double yellows and have had to wait for then to emerge from the sorting office. The parking spaces, for this reason, are as good as useless really and thus people don't tend to use them but just park on the double yellows because at least then you can leave when you have collected your parcel and not be blocked in.
Rather than park on these double yellows near the door, I tend to park on some double yellows a bit further away where I am even less likely to be a nuicance to anyone, and walk over to the door, get my parcel, etc.
I'm still "wrong" parking on the double yellows but there is nowhere else to park really, for miles. Nobody actually cares - never been told off, or booked, or inconvenienced anyone else as far as I am aware - but just making the point that you can paint double yellows all over the place if you want but it doesn't ensure people wont park there if there is little by way of alternative. It creates disfunctional behaviour in perfectly rational law abiding human beings.
Its not necessarily the case outside a school, of course - though sometimes it is. If you have a nursery with 3 parking spaces that are always full, its double yellow lines outside and all around the immediate and not so immediate vicinity, then say a parent perhaps wearing high heels and stockings ready to dash to work, hair done, its raining, there's a baby in a car seat and a toddler who bolts, 2 nursery bags, bits and bobs, coats, hats, whatever to carry...of course they will park on the double yellows for 2 minutes. It may be technically wrong but it is understandable. For that reason I tend not to get riled about it.