Not saying it is right or not, but I used to live down the road from our local ground, on street parking was mainly all we had, and match days were a nightmare so I can understand why the people might do it.
Was more annoying that there was special parking provided nearby on matchdays, but it cost �2 so people would park down our street instead. On one occasion there were 2 cars on each side of a very small parking area, so people couldn't get in or out, a note was left on windscreen.