1) it is a courtesy. unless the hospital is experiencing an internal disaster, they are still accepting patients.
2) some hospitals won't go on divert, even though there are no beds in the ER. they might go on specialty divert, but not divert for the ER.
3) even if a hospital is on divert, the patient, can still request to be taken there.
4) some crews aren't told that a hospital is on divert until they walk in the door.
5) when all hospitals in the area are on divert, then none are on divert. where I used to work, we had two hospitals; when one went on divert, the other went on divert an hour later. when I was in upstate new york, we had 5 hospitals in the city... routinely they would all go on divert, and it would be a forced rotation, so even though they were full, they were still getting patients. after all, they need to go somewhere...
BTW, I'm not saying crews should make things harder for themselves by going to an overfull hospital. However, if the patient wants to go to a hospital, they have a right to choose their destination, even if it's busy. if the crews are told it's busy, they should advise the patient of such, but patients still have the right to choose their destination.