HBO should not show this all the time and should only show it before movies and programs. I think showing it all the time is a little over the top where as only showing it occasionally gets the point across and allows viewers to actually enjoy it without it being overplayed.
I think that content warnings are important, but I do not think they should have to be shown all the time. I think it is appropriate to show them before movies and programs. If someone misses them, they could look the information up online or read the description of the show on their television.