Yes, it is. At an accident the Police will always be present. They might, or they might not be helpful. Their behavior really depends on how you are acting. If you walk around in the background and try to do your job, the police might not see you're there.
Some police officers find pressphotographers rather anoying.
The reason might be that some photographers sometimes go over the edge and are actually in the way. And only for just that picture. If you want to work for a big city newspaper, your photo-editor might be pleased with your shot. In a small local newspaper the editor might not like the picture. He won't say that you shouldn't have taken that picture, but the picture won't be printed in the newspaper.