I think the episode is horribly insensitive to anyone who has lost a loved one in much more tragic, non-self inflicted ways than how Cory Monteith died, but putting that aside, imagine the viewer who watched the episode and didn't know anything about what happened to Cory. (Unlikely sure, but possible in today's age where people don't watch news on TV and are increasingly filtering our news to our own preferences). That viewer watched the first two eps, didn't see Finn, and now all of a sudden Finn doesn't die, he's already been dead for weeks.
Without an explanation as how to he died. That viewer would be like "WTF" the entire episode. Then at the end of the episode they say "Cory Monteith and his YOB and YOD." Cue the double "WTF" and now sadness and confusion. Once the stuff about addiction comes on (Which, of course, is not Glee Canon by any means), cue triple WTF. That's the biggest problem. You can't kill a character off a television show and not explain cause of death (even if they wanted to go a different route on cause of death). Even if the real actor is dead. Especially when you decide to do an entire tribute episode.
Since we have lots of TV experts here, I'm curious if there has ever been a main character killed off in an American drama/dramedy where they don't explain/give a cause of death for the character. This can't be the first time the actor who plays a main character on a show died while the show was still going on. John Ritter comes to mind, but I didn't watch 8 Simple Rules. Recently Larry Hagman died, but "Dallas" had JR die of gunshot wounds. When Phil Hartman died (murder-suicide by his wife), Newsradio came back and had a funeral where the character died of a heart attack.